Anethole compromises human sperm function by affecting the sperm intracellular calcium concentration and tyrosine phosphorylation

Anethole is a natural anisole derivative that has been widely used in food and daily chemical industries, agricultural applications and the traditional medicine. It is closely related to aspects of daily life, and humans can easily be exposed to it. Although the reproductive toxicity of anethole was shown in the rat, its effect on human reproduction remains unknown. In this study, we examined the effect of anethole on human sperm in vitro. Different anethole doses (0.1, 1, 10, and 100 μM) were applied to ejaculated human sperm. Fertilization-essential functions, as well as the intracellular calcium concentration ([Ca²⁺]_i) and tyrosine phosphorylation, two vital factors for regulating sperm function, were measured. The results indicated that 10 and 100 μM anethole significantly reduced the motility, hyperactivation, and penetration ability of human sperm (P < 0.05) and inhibited the increase in human sperm functions induced by progesterone, a hormone essential for sperm function activation. Additionally, 10 and 100 μM anethole decreased both basal and progesterone-increased tyrosine phosphorylation, [Ca²⁺]_i, and the current of CATSPER, a cation channel of sperm predominant for Ca²⁺ influx. These results suggest that anethole inhibits human sperm functions by reducing sperm [Ca²⁺]_i through CATSPER and suppressing tyrosine phosphorylation in vitro, raising the fact that the caution is needed when overtaking anethole.     

Leflunomide (HWA 486), a novel immunomodulating compound for the treatment of autoimmune disorders and reactions leading to transplantation rejection

Leflunomide has been shown to be very effective in preventing and curing several autoimmune animal diseases. Further, this agent is as effective as cyclosporin A in preventing the rejection of skin and kidney transplants in rats. Preliminary results from patients suffering from severe cases of rheumatoid arthritis demonstrated that clinical and immunological parameters could be improved with leflunomide therapy. Mode of action studies revealed that this substance antagonizes the proliferation inducing activity of several cytokines and is cytostatic for certain cell types. In this light, we could show that tyrosine phosphorylation of the RR-SRC peptide substrate and the autophosphorylation of the epidermal growth factor (EGF) receptor were, dose dependently, inhibited by leflunomide. EGF activates the intrinsic tyrosine kinase of its receptor, which stimulates the phosphorylation of a variety of peptides, the amino acid residue in all cases is tyrosine. These results indicate that much of leflunomide's activity could be due to the inhibition of tyrosine-kinase(s), which is an important general mechanism for the proliferation of various cell types. Thus, leflunomide, which is effective against autoimmune diseases and reactions leading to graft rejection, would seem to have a mode of action separating it from known immunosuppressive drugs.     

Antibiotic-free nanotherapeutics: Hypericin nanoparticles thereof for improved in vitro and in vivo antimicrobial photodynamic therapy and wound healing

Hypericin (HY) is a naturally-occurring, potent photosensitizer. However, its lipophilicity limits its therapeutic applications. Our attempt is, thus, to develop a biodegradable nanocarrier for hypericin capable of preserving its antibacterial photoactivity. Amphiphilic block copolymers were synthesized to prepare hypericin-laden nanoparticles (HY-NPs). The antimicrobial photoactivity of HY-NPs was assessed; in vitro against biofilm and planktonic cells of methicillin resistant Staphylococcus aureus (MRSA) clinical isolates and in vivo on infected wounds in rats. Nanoparticles of 45 nm in diameter ensured higher amounts of reactive oxygen species upon irradiation. HY-NPs demonstrated superior inhibition of biofilm over planktonic cells. In vivo wound healing studies in rats revealed faster healing, better epithelialization, keratinization and development of collagen fibers when HY-NPs were applied. Determination of growth factors and inflammatory mediators in the wound area confirmed superior healing potential of nanoencapsulated hypericin suggesting that hypericin can join the era of antibiotic-free antimicrobial therapy.     

Effects of anthralin and hypericin on growth factor signalling and cell proliferation in vitro

The effect of the anthranoids, anthralin and hypericin, on epidermal growth factor receptor (EGF-R) activation and their degree of specificity was examined. Hypericin, but not anthralin, was found to inhibit binding of [¹²⁵I]-labelled epidermal growth factor (EGF) to HN5 squamous carcinoma cells that overexpress EGF-R. This effect was a result of a dose- and time-dependent reduction of EGF-R number and affinity. Neither compound directly inhibited EGF-induced tyrosine phosphorylation of the EGF-R in HN5 cells. Although anthralin and hypericin both inhibited the mitogenic effect of EGF in NR6/HER cells (IC₅₀s = 100 nM and 10 μM, respectively), they also had comparable effects on DNA synthesis in response to acidic fibroblast growth factor (aFGF) and platelet-derived growth factor (PDGF). When tested in proliferation assays using cells expressing differing numbers of EGF-R, the growth inhibitory effects of both compounds were independent of EGF-R number. We conclude that, although anthralin and hypericin both inhibit EGF signalling, they do not act specifically on the EGF-R pathway. Moreover, their mechanisms of action do not appear to be comparable.     

Inhibitors of p56lck: assessing their potential as tools for manipulating T-lymphocyte activation

The Src family tyrosine kinase p56^lck is predominantly expressed in T-lymphocytes and natural killer cells and there is an absolute requirement for p56^lck in T-cell development and activation. Consequently, there has been much effort in developing small molecule antagonists for p56^lck with a view for therapeutic use in a number of T-lymphocyte-dependent diseases. A large majority of protein kinase inhibitors are directed toward the ATP binding site and gain their specificity by exploiting adjacent topographical variations. To date, the most potent and widely studied class of Lck inhibitors are the pyrazolopyrimidines, which have proven to be valuable research tools. The development of pyrido[2,3-d]pyrimidine structures offer excellent potency and selectivity from a different structural platform and together these appear to offer the most promising scaffolds for the development of future therapeutics. SH2 domain inhibition offers the potential reward of escaping the constraints of the ATP binding site but developing potent and specific inhibitors of protein-protein interactions presents a significant challenge. The results briefly discussed in this review reveal the extent of progress toward this goal.     

Selective CCR2B receptor antagonists based on indole-2-carboxylic acid

The Src family tyrosine kinase p56^lck is predominantly expressed in T-lymphocytes and natural killer cells and there is an absolute requirement for p56^lck in T-cell development and activation. Consequently, there has been much effort in developing small molecule antagonists for p56^lck with a view for therapeutic use in a number of T-lymphocyte-dependent diseases. A large majority of protein kinase inhibitors are directed toward the ATP binding site and gain their specificity by exploiting adjacent topographical variations. To date, the most potent and widely studied class of Lck inhibitors are the pyrazolopyrimidines, which have proven to be valuable research tools. The development of pyrido[2,3-d]pyrimidine structures offer excellent potency and selectivity from a different structural platform and together these appear to offer the most promising scaffolds for the development of future therapeutics. SH2 domain inhibition offers the potential reward of escaping the constraints of the ATP binding site but developing potent and specific inhibitors of protein-protein interactions presents a significant challenge. The results briefly discussed in this review reveal the extent of progress toward this goal.     

Hypericin prolongs action potential duration in hippocampal neurons by acting on K+ channels

Background and purpose:

Synaptic deficiency is generally accepted to be involved in major depression, and accordingly classic antidepressants exert their effects through enhancing synaptic efficiency. Hypericin is one of the major active constituents of extracts of St. John''s Wort (Hypericum perforatum L.) with antidepressive actions, but little is known about its therapeutic mechanisms. Our aim was to explore whether hypericin has a modulatory effect on neuronal action potential (AP) duration by acting on voltage-gated ion channels.

Experimental approach:

We used voltage-clamp and current-clamp techniques in a whole-cell configuration to study primary cultures of neonatal rat hippocampal neurones. We measured the effects of extracellularly applied hypericin on AP duration as well as on voltage-gated Na⁺, I_A and I_K currents.

Key results:

Extracellularly applied hypericin dose-dependently increased AP duration but barely affected its amplitude. Further analysis revealed that hypericin inhibited both transient I_A and delayed rectifier I_K potassium currents. In contrast, hypericin exerted no significant effect on both Na⁺ peak current and its decay kinetics.

Conclusions and implications:

Extracellularly applied hypericin increased AP duration, which might be ascribed to its effect on I_A and I_K currents. As a small increase in AP duration could lead to a dramatic increase in synaptic efficiency, our results imply that hypericin might exert its antidepressant effects by enhancing presynaptic efficiency.     

Anti-angiogenic effects of two cystine-knot miniproteins from tomato fruit

BACKGROUND AND PURPOSE

Cystine-knot miniproteins are characterized by a similar molecular structure. Some cystine-knot miniproteins display therapeutically useful biological activities, as antithrombotic agents or tumour growth inhibitors. A critical event in the progression of tumours is the formation of new blood vessels. The aim of this work was to test two tomato cystine-knot miniproteins for their effects on endothelial cell proliferation and angiogenesis in vitro.

EXPERIMENTAL APPROACH

Two tomato cystine-knot miniproteins (TCMPs) were expressed and purified either as recombinant or as native proteins from tomato fruits. The Matrigel assay was used to investigate the effects of TCMPs on in vitro angiogenesis. Viability and proliferation of endothelial cells were tested. Extracellular signal-regulated kinase (ERK)1/2 phosphorylation was assayed in either HUVEC or A431 epidermal growth factor receptor (EGFR)-overexpressing cells treated with TCMPs. EGFR phosphorylation was tested in A431 cells.

KEY RESULTS

Both recombinant and native TCMPs inhibited in vitro angiogenesis of HUVEC cells at concentrations of 15–100 nM. The anti-angiogenic effect of TCMPs was associated with the inhibition of ERK phosphorylation. The two miniproteins did not alter the viability and proliferation of the endothelial cells.

CONCLUSIONS AND IMPLICATIONS

The anti-angiogenetic properties of TCMPs are of potential pharmacological interest because they are common and natural components of the human diet, they possess low toxicity, they are active at submicromolar concentrations, they share a common molecular structure that can be used as a molecular platform for the design of molecules with enhanced biological activity.     

LGH00031, a novel ortho-quinonoid inhibitor of cell division cycle 25B,inhibits human cancer cells via ROS generation

Aim:

To discover novel cell division cycle 25 (CDC25) B inhibitors and elucidate the mechanisms of inhibition in cancer cells.

Methods:

Cell growth inhibition was detected by MTT assay, the cell cycle was analyzed by flow cytometry, and protein expression and phosphorylation was examined by Western blot analysis.

Results:

LGH00031 inhibited CDC25B irreversibly in vitro in a dose-dependent manner, and impaired the proliferation of tumor cell lines. In synchronized HeLa cells, LGH00031 delayed the cell cycle progression at the G₂/M phase. LGH00031 increased cyclin-dependent kinase 1 (CDK1) tyrosine 15 phosphorylation and cyclin B1 protein level. The activity of LGH00031 against CDC25B in vitro relied on the existence of 1,4-dithiothreitol (DTT) or dihydrolipoic acid and oxygen. The oxygen free radical scavenger catalase and superoxide dismutase reduced the inactivation of CDC25 by LGH00031, confirming that reactive oxygen species (ROS) mediate the inactivation process in vitro. LGH00031 accelerated cellular ROS production in a dose-dependent manner, and N-acetyl cysteine (NAC) markedly decreased the ROS production induced by LGH00031. Correspondingly, the LGH00031-induced decrease in cell viability and cell cycle arrest, cyclin B1 protein level, and phosphorylation of CDK1 tyrosine 15 were also rescued by NAC that decreased ROS production.

Conclusion:

The activity of LGH00031 at the molecular and cellular level is mediated by ROS.     

Effects of halogenation on tyrosine phosphorylation and peptide binding to the SRC homology 2 domain of lymphocyte-specific protein tyrosine kinase

Phosphorylation of tyrosine residues by protein tyrosine kinases (PTK) and phosphotyrosine/Src homology 2 (SH2) domain interactions are crucial not only for signal transduction but also for regulation of PTK activity. Tyrosine residues also receive nitration and halogenation under oxidative conditions. It has been reported that nitration of tyrosine residue caused peptides to be a poor substrate for PTK and that nitrotyrosine residues could bind to SH2 domains as a phosphotyrosine mimic to activate Src family kinase. However, the effect of halogenation on tyrosine phosphorylation or SH2 domain binding is not well understood. We examined the phosphorylation of model peptides containing 3-halotyrosine or 3-nitrotyrosine using typical receptor tyrosine kinase, epidermal growth factor receptor (EGFR), and nonreceptor tyrosine kinase, lymphocyte-specific protein tyrosine kinase (Lck). The EGFR- and Lck-mediated phosphorylation was markedly inhibited by tyrosine halogenation. Iodination showed the strongest inhibition of the phosphorylation among four types of halogenation, and its inhibitory effect was stronger than that of nitration. We also examined the effect of iodination and nitration of tyrosine residues on binding to the SH2 domain of Lck, using a model peptide containing the phosphoTyr-Glu-Glu-Ile motif, which has a high affinity for the SH2 domain. The relative affinities of the modified peptides whose phosphotyrosine was substituted with unphosphorylated tyrosine, 3-nitrotyrosine, and 3-iodotyrosine, and of the model peptide were 0.024, 0.26, 1, and 16, respectively. These results suggest that tyrosine iodination may have an effect on the phosphorylation or binding to the SH2 domain similar to nitration. Tyrosine iodination possibly modulates signal transduction, with the potential impairment of cell function.     

Inhibition of insulin-like growth factor I receptor tyrosine kinase by ethanol

Ethanol inhibits insulin and insulin-like growth factor-I (IGF-I) signaling in a variety of cell types leading to reduced mitogenesis and impaired survival. This effect is associated with inhibition of insulin receptor (IR) and insulin-like growth factor-I receptor (IGF-IR) autophosphorylation, which implicates these receptors as direct targets for ethanol. It was demonstrated previously that ethanol inhibits the autophosphorylation and kinase activity of the purified cytoplasmic tyrosine kinase domain of the IR. We performed computer modeling of the ethanol interaction with the IR and IGF-IR kinases (IRK and IGF-IRK). The analysis predicted binding of alcohols within the hydrophobic pocket of the kinase activation cleft, with stabilization at specific polar residues. Using IGF-IRK purified from baculovirus-infected insect cells, ethanol inhibited peptide substrate phosphorylation by non-phosphorylated IGF-IRK, but had no effect on the autophosphorylated enzyme. In common with the IRK, ethanol inhibited IGF-IRK autophosphorylation. In cerebellar granule neurons, ethanol inhibited autophosphorylation of the apo-IGF-IR, but did not reverse IGF-IR phosphorylation after IGF-I stimulation. In summary, the findings demonstrate direct inhibition of IGF-IR tyrosine kinase by ethanol. The data are consistent with a model wherein ethanol prevents the initial phase of IRK and IGF-IRK activation, by inhibiting the engagement of the kinase activation loop.     

Sulfonylbenzoyl-nitrostyrenes: potential bisubstrate type inhibitors of the EGF-receptor tyrosine protein kinase.

The synthesis and biological activities of a series of sulfonylbenzoyl-nitrostyrene derivatives, a novel class of selective bisubstrate type inhibitors of the EGF-receptor tyrosine protein kinase, are described. The most potent derivatives inhibited the EGF-R tyrosine kinase, using angiotensin II as exogenous substrate, with IC50 values of less than or equal to 1 microM. No inhibition of the v-abl tyrosine kinase or the serine/threonine kinases PKC and PK-A was observed. In addition, active derivatives (compounds 5 and 12) effectively blocked the autophosphorylation of the EGF-R in vitro. Starting from the acids 5, 7, and 9, a series of esters, amides, and peptides was synthesized with the aim of increasing cellular penetration. Amides 14-18 showed potent antiproliferative effects using the EGF-dependent Balb/MK mouse epidermal keratinocyte cell line. Additionally, with the amide 14 inhibition of EGF-R autophosphorylation was demonstrated in the A431 cell line. CAMM studies using a computer-generated model for the transition state of the gamma-phosphoryl transfer from ATP to a tyrosine moiety and fitting experiments using the highly potent derivative 7 (IC50 value = 54 nM) support the hypothesis that the sulfonylbenzoyl group mimics a diphosphate moiety in the transition state. These results demonstrate that the rational design of tyrosine kinase inhibitors, using the inhibitory nitrostyrene moiety as a tyrosine mimic together with the sulfonylbenzoyl moiety as a diphosphate mimic, leads to highly potent and selective multisubstrate type inhibitors.     

Inhibition of benzodiazepine binding in vitro by amentoflavone,a constituent of various species of Hypericum

Flower extracts of Hypericum perforatum, Hypericum hirsutum, Hypericum patulum and Hypericum olympicum efficiently inhibited binding of [³H]flumazenil to rat brain benzodiazepine binding sites of the GABA_A-receptor in vitro with IC₅₀ values of 6.83, 6.97, 13.2 and 6.14 μg/ml, respectively. Single constituents of the extracts like hypericin, the flavones quercetin and luteolin, the glycosylated flavonoides rutin, hyperoside and quercitrin and the biflavone I3, II8-biapigenin did not inhibit binding up to concentrations of 1 μM. In contrast, amentoflavone revealed an IC₅₀ = 14.9 ± 1.9 nM on benzodiazepine binding in vitro. Comparative HPLC analyses of hypericin and amentoflavone in extracts of different Hypericum species revealed a possible correlation between the amentoflavone concentration and the inhibition of flumazenil binding. For hypericin no such correlation was observed. Our experimental data demonstrate that amentoflavone, in contrast to hypericin, presents a very active compound with regard to the inhibition of [³H]-flumazenil binding in vitro and thus might be involved in the antidepressant effects of Hypericum perforatum extracts.     

Resin immobilized synthetic peptides used to characterize phosphorylation and antigenic properties of insulin receptor autophosphorylation domains

To develop a common strategy in peptide design for kinase assay, antibody production and affinity purification, we investigated phosphorylation and antigenic properties of peptides immobilized on an aminated polyacrylic resin (Expansin ?) corresponding to autophosphorylation domains of the insulin receptor tyrosine kinase. Immobilized peptides (1143–1155) and peptide (1314–1330), designated pl 151 and p1322, respectively, were good substrates for the insulin receptor with K_m, of 0.74 and 0.78 mM. By contrast, peptide (952–963), designated p960, was poorly phosphorylated. p1151 showed distinctive behaviour as a substrate, displaying a higher basal phosphorylation, a leftward shift of the insulin dose-response curve (ED₅₀= 0.7 ng mL- ¹ insulin compared to 20 ng mL ¹ for other substrates) and an inhibition by 90% of receptor autophosphorylation (ID₅₀So = 0.5 mM). Similar substrate behaviour was observed with another tyrosine kinase, the pp60^c-src. Antibodies against P1151 and p1322 have comparable reactivity in ELISA, but the antibody against p960 was poor. While purified immunoglobulins (IgG) against both p1151 and p1322 were inhibitors of receptor autophosphorylation and kinase, in immunoprecipitation the IgG against p1151 mainly interacted with the phosphorylated receptor and that against p1322 with non-phosphorylated forms. Functional mapping of the receptor with oligoclonal 1322-antibody revealed inhibition of phosphate transfer to exogenous substrate poly(Glu,Tyr) (4:l) but not towards immobilized p1151. These data provide further support for the distinctive features of endogenous phosphorylation domain 1151. We conclude that immobilized peptides on polyacrylic resin offer a major new potential for use in kinase assays, immunization, immunoabsorbent techniques and purification of well defined oligoclonal antibodies.     

Approach to the discovery of novel,selective inhibitors of p56lck tyrosine kinase: Identification of non-hydroxylated chromones as p56lck inhibitors

The protein tyrosine kinase p56^Ick, which is expressed predominantly in lymphocytes, plays a critical role in optimal T cell activation through the T cell antigen receptor. An approach is presented for the discovery of selective p56^Ick inhibitors, which are potential immunosuppressants. A non-radioactive assay for p56^Ick tyrosine kinase activity has been developed and adapted for high volume screening. This assay does not require purified enzyme. p56^Ick in the plasma membranes of a human T cell line is purified in situ by immobilization onto the wells of a microtiter plate using an antibody specific for p56^Ick. Following the kinase reaction in the presence of test compound, autophosphorylated p56^Ick is detected with a biotinylated monoclonal antibody to phosphotyrosine. Using the approach described in this report, three simple chromones have been identified that inhibit p56^Ick autophosphorylation with low micromolar potencies and exhibit some selectivity for p56^Ick over the serine/threonine and other tyrosine kinases tested. These compounds constitute a novel group of p56^Ick tyrosine kinase inhibitors. ©1995 Wiley-Liss, Inc.     

Effects of luteolin and quercetin, inhibitors of tyrosine kinase, on cell growth and metastasis-associated properties in A431 cells overexpressing epidermal growth factor receptor

1. Flavonoids display a wide range of pharmacological properties including anti-inflammatory. Anti-mutagenic, anti-carcinogenic and anti-cancer effects. Here, we evaluated the effects of eight flavonoids on the tumour cell proliferation, cellular protein phosphorylation, and matrix metalloproteinase (MMPs) secretion. 2. Of the flavonoids examined, luteolin (Lu) and quercetin (Qu) were the two most potent agents, and significantly inhibited A431 cell proliferation with IC50 values of 19 and 21 micronM, respectively. 3. The epidermal growth factor (EGF) (10 nM) promoted growth of A431 cells (+25+/-4.6%) and mediated epidermal growth factor receptor (EGFR) tyrosine kinase activity and autophosphorylation of EGFR were inhibited by Lu and Qu. At concentration of 20 micronM, both Lu and Qu markedly decreased the levels of phosphorylation of A431 cellular proteins, including EGFR. 4. A431 cells treated with Lu or Qu exhibited protuberant cytoplasmic blebs and progressive shrinkage morphology. Lu and Qu also time-dependently induced the appearance of a ladder pattern of DNA fragmentation, and this effect was abolished by EGF treatment. 5. The addition of EGF only marginally diminished the inhibitory effect of luteolin and quercetin on the growth rate of A431 cells, treatment of cellular proteins with EGF and luteolin or quercetin greatly reduced protein phosphorylation, indicating Lu and Qu may act effectively to inhibit a wide range of protein kinases, including EGFR tyrosine kinase. 6. EGF increased the levels of matrix metalloproteinase-2 (MMP-2) and matrix metalloproteinase-9 (MMP-9), while Lu and Qu appeared to suppress the secretion of these two MMPs in A431 cells. 7. Examination of the relationship between the chemical structure and inhibitory effects of eight flavonoids reveal that the double bond between C2 and C3 in ring C and the OH groups on C3' and C4' in ring B are critical for the biological activities. 8. This study demonstrates that the inhibitory effects of Lu and Qu, and the stimulatory effects of EGF, on tumour cell proliferation, cellular protein phosphorylation, and MMP secretion may be mediated at least partly through EGFR. This study supports the idea that Lu and Qu may have potential as anti-cancer and anti-metastasis agents.     

Effects of newly synthesized benzimidazole derivatives on gastric H+/K+ ATPase

The effects of various synthetic benzimidazole derivatives on gastric H⁺/K⁺ ATPase activityin vitro were examined. The results showed that the effects of substituents on the benzimidazole ring were not significant. However, replacement of sulfoxide connecting two ring systems to sulfide resulted in a completely inactive compoundin vitro, suggesting the essential role of sulfoxide group in the inhibition. In addition, compounds with 5 or 6-membered oxacyclic substituents attached to the pyridine ring displayed the most effective inhibitory activity. Among these derivatives, AU-47 was the most potent, and detailed mechanistic studies with the compound were carried out. AU-47 inhibited gastric H⁺/K⁺ ATPase in a concentration and time dependent manner with 50% inhibition at 6 μM. The presence of sulfhydryl reducing agents or substrate analogue protected H⁺/K⁺ ATPase from the inactivation. The inhibition by AU-47 was potentiated by acid pretreatment of the compound, suggesting the structural conversion of AU-47 into a more active intermediate which was favored in acidic condition. Consistent within vitro results, AU-47 inhibitedin vivo gastric acid secretion. The results suggest that AU-47 is a relevant candidate for the development of new antiulcer agent.     

Synthetic Tyr-phospho and non-hydrolyzable phosphonopeptides as PTKs and TC-PTP inhibitors*

Tyrosine-specific protein kinases and phosphatases are important signal transducing enzymes in normal cellular growth and differentiation and have been implicated in the etiology of a number of human neoplastic processes. In order to develop agents which inhibit the function of these two classes of enzymes by interfering with the binding of their substrates, we synthesized analogs derived from the peptide EDNEYTA. This sequence reproduces the main autophosphorylation site of Src tyrosine kinases. In this work we report the synthesis, by classical solution methods, of the phosphotyrosyl peptide ED-NEYpTA as well as of three analogs in which the phosphotyrosine is replaced by a phosphinotyrosine and by two unnatural, non-hydrolyzable amino acids 4-phosphonomethyl-l -phenylalanine and 4-phosphono-l -phenylalanine. The Src peptide and its derivatives were tested as inhibitors of three non-receptor tyrosine kinases (Lyn, belonging to the Src family, CSK and PTK-IIB) and a non-receptor protein tyrosine phosphatase obtained from human T-cell (TC-PTP). The biomimetic analogues, which do not significantly affect the activity of CSK, PTK-IIB and TC-PTP, act:is efficient inhibitors on Lyn, influencing both the exogenous phosphorylation and, especially, its autophosphorylation. In particular, the Pphe derivative may provide a basis for the design of a class of inhibitors specific for Lyn and possibly Src tyrosine kinases, capable of being used in vivo and in vitro conditions. © Munksgaard 1995.