Fishing for bioactive substances from scorpionfish and some sea urchins

Venom proteins from the dorsal spine of two scorpionfish, Hypodytes rubripinnis and Synanceia verrucosa were assayed for mitogenicity and cytotoxicity. The two venoms had both mitogenic and cytotoxic activity on murine splenocytes and murine P388 leukemic cells. In H. rubripinnis, the second gel chromatographic fraction showed cytotoxic activity on P388 leukemic cells. On native PAGE, the glycoprotein isolated by concavalin A sepharose chromatography appeared to have a molecular mass of 110 kDa. In addition, two D-galactose-binding lectins (SUL-I and SUL-II) and a heparin-binding lectin (TGL-I) were purified from the globiferous pedicellariae of the toxopneustid sea urchins, Toxopneustes pileolus and Tripneustes gratilla, respectively. SUL-I (Nakagawa et al., 1999a) had mitogenic activity and cytotoxic activity but SUL-II and TGL-I did not. SUL-I did not show sequence homology to SUL-II. A hemolytic lectin with a molecular mass of 29 kDa was isolated from the coelomic fluid of T. gratilla. The hemolytic activity of the lectin was dependent on Ca2+ concentration and inhibited by lactose. The present results suggest that some species of scorpionfish and sea urchins may be novel sources for biologically active substances such as anti-tumor compounds or new lectins.     

Biochemical and physiological properties of pedicellarial lectins from the toxopneustid sea urchins

Pedicellarial lectins (SUL-I, SUL-II, and TGL-I) were purified from the toxopneustid sea urchins, Toxopneustes pileolus and Tripneustes gratilla using gel filtration chromatography, affinity chromatography, and reverse-phase HPLC. SUL-I (Nakagawa et al., 1996) and SUL-II from the large globiferous pedicellariae of T. pileolus are D-galactose-binding lectins with molecular masses of 32 kDa and 23 kDa, respectively; while TGL-I from the globiferous pedicellariae of T. gratilla is a Ca(2+)-independent heparin-binding lectin with a molecular mass of 23 kDa. SUL-I induced mitogenic stimulation on murine splenocytes but TGL-I did not. At higher dose ranges SUL-I exhibited inhibitory effects on the cells. The dual response to SUL-I was effectively inhibited by D-galactose. SUL-I enhanced norepinephrine-induced contraction of isolated rat mesenteric artery with endothelium. When endothelium was removed from the artery, acetylcholine did not relax the norepinephrine-induced contraction. In the same artery the enhancing effect of the contraction by SUL-I was abolished, suggesting that SUL-I acts on the endothelium of mesenteric artery, and may release prostanoids. The present results suggest an extracellular function for SUL-I that may have wide-ranging effects in physiological processes. The primary role of pedicellarial lectins from T. pileolus and T. gratilla might be defense against a foreign body.     

Isolation and Molecular Characterization of Two Lectins from Dwarf Elder (Sambucus ebulus L.) Blossoms Related to the Sam n1 Allergen

Sambucus species contain a number of lectins with and without antiribosomal activity. Here, we show that dwarf elder (Sambucus ebulus L.) blossoms express two d-galactose-binding lectins that were isolated and purified by affinity chromatography and gel filtration. These proteins, which we named ebulin blo (A-B toxin) and SELblo (B-B lectin)—blo from blossoms—were subjected to molecular characterization and analysis by MALDI-TOF mass spectrometry and tryptic peptide fingerprinting. Both lectins share a high degree of amino acid sequence homology with Sambucus lectins related to the Sam n1 allergen. Ebulin blo, but not SELblo, was highly toxic by nasal instillation to mice. Overall, our results suggested that both lectins would belong to an allergen family exemplified by Sam n1 and could trigger allergy responses. Furthermore, they raise a concern about ebulin blo toxicity.     

Purification and structural characterization of lectins from the cnidarian Bunodeopsis antilliensis [corrected].

Purification and characterization of two different lectins from the Mexican anemone Bunodeopsis antilliensis are reported. These two lectins named Bunodeopsis antilliensis agglutinin-A (BAA-A) and -B (BAA-B) presented the following characteristics: BAA-A was resolved as a component, with haemagglutinating activity for human blood type A (N-acetylgalactosamine-galactose-fucose), with a molecular weight of 28,900 obtained by means of mass spectrometry, showed an isoelectric point of 5.04 with a higher carbohydrate specificity for N-acetylgalactosamine (GalNAc). The analysis of the N-terminal revealed it is related to phosphoesterase and GTP binding protein. BAA-B mainly active with human blood type B (galactose-galactose-fucose) was resolved into three fractions (BAA-B1-3). Their molecular weight were: BAA-B(1) 39,350, BAA-B(2) 28,300 and BAA-B(3) 17,550. The estimated isoelectric points were 8.05, 4.66 and 6.60, respectively. Only fraction 3 exhibited haemagglutinating activity with a higher carbohydrate specificity for galactose and mannose. The analysis of the N-terminal pointed out it is related with phospholipase A(2). We suggest these lectins could be related to a feeding strategy[corrected].     

Inhibition of lung tumor colonization and cell migration with the disintegrin crotatroxin 2 isolated from the venom of Crotalus atrox.

Disintegrins are low molecular weight proteins (4-15 kDa) with an RGD binding region at their binding loop. Disintegrin and disintegrin-like proteins are found in the venom of four families of snakes: Atractaspididae, Elapidae, Viperidae, and Colubridae. This report describes the biological activity of a disintegrin, crotatroxin 2, isolated by a three-step chromatography procedure from the venom of the Western diamondback rattlesnake (Crotalus atrox). The intact molecular mass for crotatroxin 2 was 7.384 kDa and 71 amino acids. Crotatroxin 2 inhibited human whole blood platelet aggregation with an IC(50) of 17.5 nM, inhibited cell (66.3p) migration by 63%, and inhibited experimental lung tumor colonization in BALB/c mice at 1000 microg/kg. Our data suggest that while crotatroxin 2 inhibits platelet aggregation, cancer cell migration, and lung tumor colonization, it is done via different integrins.     

Modulation of mistletoe (Viscum album L.) lectins cytotoxicity by carbohydrates and serum glycoproteins

Three D-galactose and/or N-acetyl-D-galactosamine specific mistletoe lectins, ML I, ML II and ML III, were purified by affinity chromatography followed by cation exchange chromatography. These lectins were toxic for Molt 4 cells in culture at concentrations in the pg/ml range, ML III being the most cytotoxic. Carbohydrates able to bind to the B-chain of these lectins inhibited their toxic activity. The digalactosides Gal beta 1,2Gal beta-allyl and Gal beta 1,3Gal beta-allyl were 60 and 30 times, respectively, more potent than D-galactose in their ability to protect the cells from the ML I cytotoxicity. N-acetyl-D-galactosamine and rho-nitrophenyl N-acetylgalactosamine protected mainly from the toxic effects of ML II and III. Protection from cytotoxicity varied in the same order as the affinity of the tested carbohydrates for lectins. Serum glycoproteins particularly haptoglobin, but also alpha 1-acid glycoprotein and transferrin, notably inhibited the cytotoxicity of the lectins. This effect was due to the binding of lectin to the sugar moiety of the glycoprotein because deglycosylated haptoglobin did not have a protective activity on Molt 4 cells. Inhibition of the cytotoxicity of lectins by serum glycoproteins explains why mistletoe extracts containing lectins can be administered to cancer patients without harmful effects.     

Properties and interaction of heterologously expressed glutamate decarboxylase isoenzymes GAD(65kDa) and GAD(67kDa) from human brain with ginkgotoxin and its 5'-phosphate.

Two isoforms of glutamate decarboxylase (GAD(65kDa) and GAD(67kDa)) from human brain, which had previously been overexpressed in Escherichia coli as fusion proteins containing a glutathione-S-transferase domain, were purified by affinity chromatography on glutathione Sepharose 4B. Both isoforms were also expressed in Saccharomyces cerevisiae. After modification of a HPLC based assay, the enzymes were characterized with respect to their biochemical properties. Comparison of kinetic data, pH, and temperature optima as well as of the mode of interaction with pyridoxal phosphate as a cofactor revealed several significant differences between the two isoenzymes reflecting their somewhat different physiological and molecular features. Investigation of the influence of 4'-O-methylpyridoxine (ginkgotoxin) (1), a neurotoxin occurring in Ginkgo biloba L., on the different isoenzymes, indicates that the phosphorylated form of the toxin, 4'-O-methylpyridoxine-5'-phosphate (2), decreases GAD(65kDa) activity, although in unphysiologically high concentrations, whereas GAD(67kDa) activity seems to be hardly affected.     

Purification and characterization of human kidney cytosolic cysteine conjugate beta-lyase activity

The central role of cysteine conjugate beta-lyase (beta-lyase) in the bioactivation of nephrotoxic halogenated hydrocarbons and the possibility of human exposure to these chemicals has focused interest on the beta-lyase from human kidney. Human kidney tissue was collected as surgical waste material, and subcellular fractions were isolated by differential centrifugation. Human beta-lyase activity, determined with S-(2-benzothiazolyl)-L-cysteine (BTC) as the substrate, was present in the cytosolic, mitochondrial, and microsomal fractions, but was highest in the cytosolic fraction. Activity in human kidney cytosol was about 10% of that present in rat kidney cytosol. Human kidney cytosolic beta-lyase activity, with BTC as the substrate, was not stimulated by pyridoxal phosphate or by exogenous 2-keto acids. Cytosolic beta-lyase activity was purified 280-fold with a yield of 12% from human kidneys unsuitable for transplantation. The beta-lyase activity copurified with cytosolic glutamine transaminase K and exhibited a molecular mass of 85 kDa on a Sephacryl 5300 column and a subunit molecular mass of 45 kDa by gel electrophoresis. Both BTC and its homocysteine analogue S-(2-benzothiazolyl)-L-homocysteine were excellent substrates, exhibiting Km and kcat values of 0.97 mM and 2.78 mM and 9.35 min-1 and 6.90 min-1, respectively. beta-Lyase activity was inhibited by aminooxyacetic acid, indicating that the human cytosolic enzyme contains pyridoxal phosphate, and by the nephrotoxins S-(1,2-dichlorovinyl)-L-cysteine and S-(1,2-dichlorovinyl)-L-homocysteine, which serve as alternative substrates.(ABSTRACT TRUNCATED AT 250 WORDS)     

A novel platelet glycoprotein Ib-binding protein with human platelet aggregation-inhibiting activity from Trimeresurus jerdonii venom

Platelet glycoprotein Ib (GPIb) is a primary adhesion receptor and involved in platelet-related disorders. However, it is difficult to study GPIb-specific platelet stimulation using physiological ligands in vivo. GPIb-binding snake C-type lectins (snaclecs) are useful tools for exploring GPIb in vitro because they act on platelets differently. In the present study, a novel GPIb-binding snaclec, named jerdonibitin, was purified, molecular cloned and characterized from Trimeresurus jerdonii venom. On SDS-polyacrylamide gel electrophoresis, it showed a single band with an apparent molecular weight of 25 kDa under non-reducing conditions and two distinct bands with apparent molecular weights of 15 kDa (α-subunit) and 13 kDa (β-subunit) under reducing conditions. The cDNA sequences of each subunit of jerdonibitin were identified and both deduced amino acid sequences were confirmed by N-terminal protein sequencing and trypsin-digested peptide mass fingerprinting of MALDI-TOF. Sequence alignment showed that jerdonibitin is a snaclec and has sequence similarity with TSV-GPIb-BP (a GPIb-inhibitory snaclec). Jerdonibitin dose-dependently inhibited platelet aggregation induced by ristocetin or low-dose thrombin, but not by high-dose thrombin. The GPIbα was detected by affinity chromatography on jerdonibitin. In vivo, jerdonibitin also dose-dependently induced thrombocytopenia of mice and platelet counts remained at very low level after 18 h intravenous injection. In summary, a novel GPIb-inhibitory snaclec was molecular cloned and characterized, which might provide insights into investigation of how GPIb-inhibitory snaclecs work and development of new antiplatelet agents.     

Purification and characterization of a lectin from Euphorbia nivulia Buch. Ham. latex

A lectin from Euphorbia nivulia Buch. Ham. latex was purified by affinity chromatography on galactose-Sepharose-6B. Euphorbia nivulia latex (ENL-L) was resolved into four lectin fractions on CM-Sephadex C-50 column. All the four fractionated lectins were found to be homogeneous. One of the major fractionated lectins (ENL-L-IV) was further characterised. ENL-L-IV shows a single band on PAGE at pH 4.5 and SDS-PAGE. Both ENL-L and ENL-L-IV are glycoproleins containing 13.6% and 9% of carbohydrates respectively. ENL-L has a molecular weight of 45 700 and has two subunits with M_r of 19 000 and 22 800 and a sedimentation coefficient of 4S. ENL-L-IV has a molecular weight of 44 000 and appears to be a dimer with subunits of M_r of 22 000. Ultracentrifugation studies showed that it has a M_r of 29 000 and a sedimentation coefficient of 4.8S. Both ENL-L and ENL-L-IV are galactose specific and are also inhibited by galactose derivatives and its oligosaccharides. with increased β-anomeric specificity. They are found to be mitogenic against human lymphocytes and blood group non-specific. ENL-L-IV was found to be immunologically identical with ENL-L and other fractionated ENL lectins.     

人精子甘露糖受体的部分特性研究

目的分离纯化人精子甘露糖受体并测定其分子量和等电点。方法用亲和层析法分离纯化人精子甘露糖受体,十二烷基磺酸钠-聚丙烯酰胺凝胶电泳法测定其分子量,等电聚焦电泳法测定其等电点。结果人精子甘露糖受体的分子量为66.3 ku,等电点为pH=5.1。结论用亲和层析法分离得到人精子甘露糖受体,并测得其分子量和等电点,可为后续开展人精子甘露糖受体有关理化特性和医用价值的研究积累有价值的资料。     

Purification and characterization of charantin,a napin‐like ribosome‐inactivating peptide from bitter gourd (Momordica charantia) seeds

Abstract: A peptide designated charantin, with a molecular mass of 9.7 kDa, was isolated from bitter gourd seeds. The procedure comprised affinity chromatography on Affi‐gel blue gel, ion‐exchange chromatography on Mono S and gel filtration on Superdex 75. The N‐terminal sequence of charantin exhibited marked similarity to that of the 7.8‐kDa napin‐like peptide previously isolated from bitter gourd seeds. Charantin inhibited cell‐free translation in a rabbit reticulocyte lysate system with an IC₅₀ of 400 nm , a potency lower than that of the previously reported small ribosome‐inactivating protein γ‐momorcharin (IC₅₀ = 55 nm ) which also exhibited an abundance of arginine and glutamate/glutamine residues. Charantin reacted positively in the N‐glycosidase assay, yielding a band similar to that formed by the small ribosome‐inactivating proteins γ‐momorcharin and luffin S.     

Influence of carbohydrates on the cytotoxicity of an aqueous mistletoe drug and of purified mistletoe lectins tested on human T-leukemia cells

Partially and highly purified lectins from Viscum album L. (mistletoe) cause a dose-dependent decrease of viability of human leukemia cell cultures, MOLT-4, after 72 h treatment. The LC50 of the partially purified lectin was 27.8 ng/ml, of the highly purified lectin 1.3 ng/ml. Compared to the highly purified lectin a 140-fold higher protein concentration of an aqueous mistletoe drug was required to obtain similar cytotoxic effects on MOLT-4 cells. Cytotoxicity of the highly purified lectin was preferentially inhibited by D-galactose and lactose, cytotoxicity of the mistletoe drug and the partially purified lectin were preferentially inhibited by lactose and N-acetyl-D-galactosamine (GalNAc). Two lectin fractions with almost the same cytotoxic activity on MOLT-4 cells but with different carbohydrate affinities were isolated by affinity chromatography from the mistletoe drug: mistletoe lectin I with an affinity to D-galactose and GalNAc and mistletoe lectin II with an affinity to GalNAc. The lectin fractions and the mistletoe drug inhibited protein synthesis of MOLT-4 cells stronger than DNA synthesis. Furthermore a subpopulation of MOLT-4, resistant to cytotoxic doses of both the mistletoe drug and the mistletoe lectins, was shown to exhibit a reduced amount of GalNAc and N-acetyl-D-glucosamine in their cellular glycoproteins which are probably responsible for the binding of the cytotoxic lectins. These results indicate that lectins are the main toxins in the mistletoe drug.     

Comparative study of protein molecular weights by size-exclusion chromatography and laser-light scattering

High-performance size-exclusion chromatography (SEC) based on UV-Vis detection is a relative technique for molecular weight determination whereas procedure based on multi-angle laser light scattering (MALLS) is both rapid and absolute. The two methods using recombinant human growth hormone (rHGH) and β-lactoglobulin samples were compared. A calibration curve for the chromatographic system was generated based on standard proteins and the data were fitted by least squares to a third order polynomial model. The molecular weight from the conventional SEC method for both proteins was higher than the reported values. The molecular weight of rHGH from MALLS was 23.1±0.57 and 21.2±0.80 kDa using differential refractive index (SEC-MALLS/RI) and UV (SEC-MALLS/UV-Vis) detectors as mass detectors. Both values agree, within experimental error with the molecular weight sequence of rHGH, 22.1 kDa. In contrast, the molecular weight from LS for β-lactoglobulin was 22.5±0.55 kDa by SEC-MALLS/RI and 23.0±1.22 kDa by SEC-MALLS/UV-Vis, respectively, values always higher than those supplied by the manufacturer, 18.4 kDa. The reproducibilty of the SEC-MALLS/UV-Vis method versus the SEC-MALLS/RI method was performed using the concordance correlation coefficient. The method′s reproducibility was accepted by assuming a precision of 98% and a 1% loss in precision.     

Immunological characterization of dehydroepiandrosterone sulfotransferase from human liver and adrenal.

Dehydroepiandrosterone sulfotransferase (DHEA-ST), a steroid sulfotransferase (ST), has recently been purified from human liver cytosol and partially characterized. DHEA-ST has a subunit molecular mass of 35 kDa and is responsible for the majority of the sulfation of steroids and bile acids in the liver. For these studies, polyclonal antibodies to human liver DHEA-ST were raised in rabbits. The anti-human liver DHEA-ST antibodies were used to characterize the immunoreactivity of DHEA-ST in human liver and to study the relationship of human adrenal DHEA-ST to the liver form of the enzyme. Immunoblot analysis of several different human liver cytosol samples with the rabbit anti-human liver DHEA-ST antiserum detected only a single 35-kDa protein in each liver. Anti-human liver DHEA-ST antibodies also did not react with either form of phenol sulfotransferase (PST), P-PST or M-PST, present in human liver cytosol. DHEA-ST activity was purified from the 100,000 x g supernatant fraction of human adrenal tissue by DEAE-Sepharose CL-6B chromatography and 3',5'-diphosphoadenosine-agarose affinity chromatography. Human adrenal DHEA-ST was shown to have a molecular mass of 35 kDa, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunoblot analysis of human adrenal cytosol revealed that the anti-human liver DHEA-ST antibodies reacted specifically with the 35-kDa subunit of DHEA-ST. The apparent Km values for DHEA and 3'-phosphodenosine-5'-phosphosulfate obtained with human adrenal DHEA-ST were 1.0 microM and 1.6 microM, respectively. Adrenal DHEA-ST demonstrated the same pattern of reactivity towards different steroid substrates as did human liver DHEA-ST, and neither form of DHEA-ST was found to sulfate cortisol. The results of this study suggest that DHEA-ST is the major steroid ST present in human liver and adrenal tissue and that the physical, biochemical, and kinetic properties of adrenal DHEA-ST are similar if not identical to those of the liver form of the enzyme.     

The antihemorrhagic factor, erinacin, from the European hedgehog (Erinaceus europaeus), a metalloprotease inhibitor of large molecular size possessing ficolin/opsonin P35 lectin domains.

From muscle extracts of the European hedgehog, Erinaceus europaeus, an antihemorrhagic factor, erinacin, was purified by ammonium sulfate precipitation followed by chromatography on DEAE-cellulose, hydroxylapatite and gel filtration columns. A purification of approx. 1400-fold was achieved with an overall yield of 21% in antihemorrhagic activity. The molecular weight of erinacin determined by gel filtration was approx. 1000 kDa. SDS-PAGE of erinacin under reducing conditions indicates that it consists of two types of subunits, alpha and beta, with molecular weights of 37 and 35 kDa, respectively, in a ratio of 1:2. In the presence of 6 M guanidine-HCl, erinacin dissociates into alpha-subunits and beta-subunit decamers. From these results the subunit assembling of erinacin has been formulated as alpha(10).2beta(10). The molecular weight of the subunits and of the beta-subunit decamer was confirmed by MALDI-TOF mass spectrometry. Erinacin inhibits the hemorrhagic and proteolytic activity of the major hemorrhagic metalloprotease from the venom of Bothrops jararaca. Complete inhibition was achieved in an equimolar mixture of inhibitor and enzyme suggesting an equimolar complex. Erinacin is not inhibiting serine proteases such as trypsin and chymotrypsin, it was characterized to be a metalloprotease inhibitor. In electronmicroscopy, flower bouquet-like structures characteristic for some animal lectins were observed. Amino acid sequence analysis indicated that both subunits are almost identical and are composed of common amino terminal, collagen- and fibrinogen-like domains homologous to proteins of the ficolin/opsonin P35 lectin family.     

Purification and biological activity of acidic polysaccharide from leaves of Thymus vulgaris L

Polysaccharides are involved in biological responses and can activate complement system, which plays an important role in primary host defense mechanisms. We investigated anticomplementary activities from spice plants and selected thyme (Thymus vulgaris L.) as a potent complementary activator. Acidic polysaccharide (TV-3-IIIA-IIa) purified from the hot-water extract of thyme leaves by DEAE-Toyopearl 650C, Butyl-Toyopearl 650M and Sepharose CL-6B column chromatography and preparative HPLC. The purified polysaccharide, TV-3-IIIA-IIa showed potent anticomplementary activity via classical and alternative pathway with the increase proportional to dosage. TV-3-IIIA-IIa seemed to be a homogenous polymer from the results of HPLC and its molecular mass was estimated as 180 kDa. TV-3-IIIA-IIa mainly consisted of galacturonic acid (44.8 mol%), glucuronic acid (16.7 mol%), arabinose (11.1 mol%), rhamnose (9.2 mol%), galactose (8.9 mol%) and small amounts of glucose, xylose, mannose and fucose. By methylation analysis and reactivity to beta-glucosyl Yariv reagent, TV-3-IIIA-IIa was assumed to contain small amounts of type II arabinogalactan and large amounts of pectin-like polysaccharides in the structure. Based upon these results, TV-3-IIIA-IIa was suggested to be a complement activator.     

Acid phosphatases from the liver of Labeo rohita: purification and characterization

Low molecular weight acid phosphatase (LM-ACP) peak 2 (the isoenzyme corresponding to isoform 2, IF-2) from the liver of fish Rahu (Labeo rohita) was purified to homogeneity. 900 times purification resulted with specific activity of 35 U/mg of protein and recovery of 0.2%. The enzyme was found homogeneous on sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). Molecular weight of 18 killo Daltons (kDa) was obtained. The peak 1 isoenzyme corresponding to isoform1 (IF-1) was partially purified about 160 times with specific activity of 7 U/mg of protein. Major protein band corresponding to 18 kDa was seen along with other protein faint bands. High molecular weight acid phosphatase (HM-ACP) was also partially purified. The molecular weight was estimated to be a 100 kDa by gel filtration on Sephadex G-100. LM-ACP isoenzymes and HM-ACP enzyme were studied for their substrate specificity, sensitivity to inhibitors or activators and other kinetic parameters. LM-ACP isoenzymes were not inhibited by tartrate and fluoride but were inhibited by sulfhydryl reagent whereas high molecular weight enzyme was strongly inhibited by fluoride and tartrate. Phosphate vanadate and molybdate inhibited both types of enzymes competitively, but their action was more pronounced in HM-ACP enzyme. LM-ACP was effectively activated by purine compounds whereas HM-ACP was not. LM-ACP showed strict substrate specificity while HM-ACP showed broad substrate specificity. The two types of acid phosphatases also differed in their rate of hydrolysis of alpha-naphthyl phosphate and beta-glyerophosphate.     

Human kidney thiopurine methyltransferase. Photoaffinity labeling with S-adenosyl-L-methionine.

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of heterocyclic and aromatic sulfhydryl compounds such as the thiopurine drug 6-mercaptopurine (6-MP). TPMT activity in human tissue is regulated by a common genetic polymorphism, and "pharmacogenetic" variation in TPMT activity is an important factor in individual differences in thiopurine drug metabolism, toxicity and therapeutic efficacy. Human renal tissue contains two isozymes of TPMT, Peak I and Peak II, that can be separated by ion exchange chromatography. Our experiments were performed to determine whether S-adenosyl-L-methionine (Ado-Met), the methyl donor for the TPMT reaction, could be used as a photoaffinity ligand for these isozymes as one step in the study of the molecular basis for the TPMT genetic polymorphism. When [3H-methyl]Ado-Met and partially purified preparations of either isozyme of human kidney TPMT were exposed to ultraviolet light at 254 nm, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, a 35 kDa protein was the predominant species that was radioactively labeled. The same 35 kDa protein was photoaffinity labeled with [14C-carboxyl]Ado-Met, demonstrating that labeling involved covalent binding of Ado-Met rather than methylation of the protein. TPMT enzymatic activity co-eluted with the 35 kDa protein during sequential DEAE ion exchange, gel filtration and hydroxylapatite chromatography. Inhibitors of TPMT enzymatic activity including S-adenosyl-L-homocysteine, sinefungin, 6-methylmercaptopurine and 3,4-dimethoxy-5-hydroxybenzoic acid inhibited photoaffinity labeling of the 35 kDa protein in preparations of both TPMT Peak I and Peak II isozymes in a concentration-dependent fashion, as did 6-MP, the methyl acceptor substrate for the TPMT reaction. All of these results were compatible with the conclusion that the 35 kDa protein was TPMT. Photoaffinity labeling of TPMT with [3H]Ado-Met should make it possible to purify the enzyme to homogeneity and to study amino acid sequences at or near its active site.