PEGylation enhancement of pH stability of uricase via inhibitive tetramer dissociation

Objectives Previously, PEGylated uricase was demonstrated to maintain catalytic activity at pH 5.8, the isoelectric point of uricase, where native uricase ceases to function. To find out whether PEGylation could enhance pH stability of uricase, the enzyme activity to pH curve was completely characterized. Methods Complete characterization of the enzyme activity to pH curve, indicating an inverted bell‐shaped relationship not previously documented, is presented. PEGylation enhancement of uricase stability at a pH lower than that commonly found in the liver, can be explored by dynamic dissociation of uricase using ultrafiltration and size‐exclusion chromatography. Key findings The results suggest the role of PEGylation in enhanced pH stability is via inhibition of subunit disintegration. The mechanism of this effect is characterized by the wrapping of PEG chains around uricase, providing a flexible shell preventing subunit disintegration. The presence of notable PEGylation‐induced changes in uricase supports this mechanism and include improved enzyme‐substrate affinity and elevated thermal stability. Conclusions Characterization of PEGylated uricase provides a basis for the rational design of therapeutic PEGylated proteins.     

Production and Optimization of Site-Specific monoPEGylated Uricase Conjugates Using mPEG-Maleimide Through RP–HPLC Methodology

Purpose

Uricase (Uc), a therapeutic enzyme, is widely used in its PEGylated form to treat hyperuricemia and is largely manufactured by means of random/first generation PEGylation approach. Currently available randomly PEGylated uricase conjugates exhibit inadequacies like reduced uricolytic activity, risk of inducing immunogenic reactions, lack of selectivity, and molecular heterogeneity. In the present study, site-specific/second generation PEGylation strategy involving modification of specific and rare amino acids by means of terminally functionalized PEG polymers was applied.

Methods

Uricase was conjugated with methoxypolyethyelenglycol-maleimide (mPEG-mal) by means of thiol PEGylation to synthesize monoPEGylated uricase conjugates. For enhancing the yield of monoPEGylated uricase conjugates, response surface methodology was employed to determine the yield of monoPEGylated conjugates using reverse phase high performance liquid chromatography. Using the optimized conditions, the developed method was validated for the production of monoPEGylated uricase conjugates which were further purified by size exclusion fast protein liquid chromatography (SE-FPLC). The molecular weights of the purified conjugates were determined by sodium dodecyl sulfide polyacrylamide gel electrophoresis (SDS-PAGE).

Results

The optimum values of reaction conditions were determined as 1:12 concentration ratio of Uc to mPEG-mal, 2.76 kDa as mPEG-mal molecular weight and 3.55 mM EDTA concentration which resulted in a very high conjugate yield of 95.16 %. The conjugate synthesized using the optimized method retained a residual uricolytic activity of 84 % and a thiol group modification extent of 68.3 %.

Conclusion

The PEGylation reaction was optimized using OVAT and statistical methods. Using the optimized conditions very high yield of conjugates were obtained and RP–HPLC method was used to quantify the PEGylated uricase.

     

Therapeutic proteins: a comparison of chemical and biological properties of uricase conjugated to linear or branched poly(ethylene glycol) and poly(N-acryloylmorpholine)

Uricase from Bacillus fastidiosus (UC) was covalently linked to linear PEG (PEG-1) (Mw 5 kDa), branched PEG (PEG-2) (Mw 10 kDa) and to poly(N-acryloylmorpholine) (PAcM) (Mw 6 kDa). The conjugation of UC with linear PEG and PAcM was accompanied by complete loss of enzymatic activity but, if uric acid as site protecting agent was included in the reaction mixture, the conjugate protein retained enzymatic activity. On the other hand, the modification with PEG-2 gave a conjugate that also maintained enzymatic activity in the absence of any active site protection. This behaviour must be related to hindrance of the branched polymer in reaching the enzyme active site. The UC conjugates exhibited increased resistance to proteolytic digestion while minor variations in the inhibitory constant, optimal pH, heat stability, affinity for substrate, were observed. Pharmacokinetic investigations in mice demonstrated increased residence time in blood for all the conjugates as compared with native uricase. Uricase conjugated with linear PEG was longer lasting in blood UC derivative, followed by branched PEG and the PAcM conjugates. Unconjugated uricase was rapidly removed from circulation. All these data are in favour of the use of the less known amphiphilic polymer PAcM as an alternative to PEGs in modification of enzymes devised for therapeutic applications.     

Synthesis,Characterization and In Vivo Efficacy of PEGylated Insulin for Oral Delivery with Complexation Hydrogels

Purpose  This work evaluated the feasibility of combining insulin PEGylation with pH responsive hydrogels for oral insulin delivery. Methods  A mono-substituted PEG–insulin conjugate was synthesized and purified. The site of conjugation was determined by MALDI-TOF MS. Uptake and release of PEGylated insulin was performed in complexation hydrogels to simulate oral dosing. The bioactivity of the conjugate and PK/PD profile was measured in vivo in rats. Results  PEGylation was confirmed to be specifically located at the amino terminus of the B-chain of insulin. Higher loading efficiency was achieved with PEGylated insulin than regular human insulin in pH responsive hydrogels. The release of PEGylated insulin was lower than that of human insulin at all pH levels considered. Full retention of bioactivity of the PEG–insulin conjugate was confirmed by intravenous dosing while subcutaneous dosing exhibited a relative hypoglycemic effect 127.8% that of human insulin. Conclusions  Polyethylene glycol conjugated specifically to the amino terminus of the B-chain of insulin maintained the bioactivity of the protein and significantly extended the duration of the hypoglycemic effect. Used in combination with pH responsive hydrogels, PEGylated insulin has significant potential for oral delivery.     

蛋清溶菌酶部分酶学性质及酶活性的影响因素研究

目的探讨将蛋清溶菌酶制成液体制剂的可行性。方法通过改变不同影响因素测定蛋清溶菌酶的活性,观察几种常用辅料对蛋清溶菌酶活性的影响。结果酶活性在pH 6.0~6.5最强,且在pH 5~7范围内较稳定;在25~65℃范围内随着作用温度的升高酶的活性增强,但温度太高则变性失活;Na+、K+对其活性有轻微激活作用,Mn2+、Mg2+对溶菌酶活性无明显影响,Co2+、Ca2+、Cu2+、Fe2+、Zn2+使溶菌酶的活性下降;吐温20、吐温80、甘油溶液对酶活有抑制作用;EDTA-2Na在0.000 5%~0.300 0%浓度范围内对溶菌酶活性具有激活作用,浓度继续增加反而有抑制活性作用。结论初步试验结果表明溶菌酶可以制成合适的液体制剂。     

几种因素在L-天冬酰胺酶化学修饰中对酶活力及抗原性的影响

为了寻求在较好地保持酶活力的同时解除L-天冬酰胺酶抗原性的方法,采用不同分子量的乙酸酐、右旋糖酐和单甲氧基聚乙二醇,作为修饰剂和不同的修饰方法对该酶进行了化学修饰。结果表明在保持酶活性和降低抗原性方面,大分子修饰剂右旋糖酐、单甲氧基聚乙二醇优于小分子乙酸酐,底物保护修饰优于直接修饰;活化PEG,优于活化PEG₁。在底物保护下的PEG,修饰酶其抗原性完全解除的同时,酶活力保持在30%以上。     

THE PH DEPENDENCE OF BINDING OF α, α‘-DIBROMO-P-XYLENESULFONIC ACID TO LYSOZYME

The chemical modification of lysozyme (I) has been accomplished with α,α'-dibromo-p-xylenesulfonic acid (DBX) at five different pH values. I was alkylated by DBX at room temperature (28°C) with decrease in enzyme activity. The rate of inactivation depended upon the pH at which alkylation was carried out. The highest rate was seen at alkaline pH values; the lowest at more acidic pH values. Amino acid analyses showed that two lysines and two tryptophan residues had been modified at pH 9; two lysines, one tryptophan and one methionine had reacted at pH 8. A histidine residue was bound at pH 6.5 together with a tryptophan residue. At the lower pH values (2.7, 4.5, 6.5), alkylation occurred with a single tryptophan residue each. Fluorescence and CD data both ruled out the participation of tryptophans 62 or 108. Labeling experiments showed that two residues of DBX-³⁵S were bound per molecule of I at both pH 9 and pH 8; one residue of DBX was bound per molecule of I at the other pH values. Sedimentation coefficients were characteristic of native lysozyme. The stoichiometry of binding and residue modification indicated that intra-molecular cross links were established. The pH dependence of the cross-linking provides means to measure several allowed intramolecular distances. The results presented here are consistent with the existence of side chain motion in lysozyme.     

Novel alginate gel microspheres produced by impinging aerosols for oral delivery of proteins

Lysozyme and insulin were encapsulated in alginate gel microspheres using impinging aerosols method. High loadings of around 50% weight/dry microspheres weight were obtained with encapsulation efficiencies of at least 48%. Environmental scanning electron microscopy revealed smooth spherical hydrated microspheres (30–60?µm) in diameter. No lysozyme or insulin release was measured in simulated gastric fluid (HCl, pH 1.2, 37°C). Total insulin release occurred in simulated intestinal fluid (SIF; phosphate buffer saline, pH 7.4, 37°C) in 8?h following 2?h incubation in SGF and was found to retain 75% activity using the ARCHITECT® assay. Lysozyme was released completely in SIF in 10?h following 2?h incubation in SGF and was found to exhibit at least 80% bioactivity using the Micrococcus lysodeikticus assay. The absence of protein release in HCl and the retention of high levels of biological activity demonstrate the potential of alginate gel microspheres, for improving oral delivery of biopharmaceuticals.     

溶菌酶的处方设计前稳定性研究

目的考察pH值、温度、保护剂、有机溶剂对溶菌酶物理稳定性和生物活性的影响,为其缓释注射剂的处方学研究提供依据。方法以HPLC法测定溶液中溶菌酶的含量,以比浊法测定溶菌酶的生物活性。结果在pH4．0、pH10．0条件下溶菌酶的物理稳定性较好,pH4．0条件下溶菌酶可保持较高的生物活性,pH1．0的条件会完全破坏溶菌酶的溶菌活性;低温条件有利于保持溶菌酶的物理稳定性和生物活性;甘露醇对溶菌酶的稳定性具有先增强后减弱的作用;有机溶剂会严重破坏溶菌酶的物理稳定性,但仍保留着一部分溶菌活性。结论pH值、温度、保护剂、有机溶剂对溶菌酶的物理稳定性和生物活性具有明显影响。     

Control of pharmaceutical properties of soybean trypsin inhibitor by conjugation with dextran. I: Synthesis and characterization

The Kunitz-type soybean trypsin inhibitor (STI), a model protein, was conjugated with dextran (Mw, approximately 9900; STI-D), and its physicochemical and biochemical properties were studied to develop a novel delivery system for a protein drug. Conjugation was carried out using periodate oxidation, and cyanogen bromide, carbodiimide, cyanuric chloride, epichlorhydrin, and N-succiniimidyl-3-(2-pyridyldithio)propionate (SPDP) reagent methods. Dextran was conjugated to STI at a molar ratio of 1.5 to 4.6, but the degree of modification, as well as yield and contamination extent of unreacted STI and dextran, varied with the method of synthesis. Gel filtration and electrophoresis confirmed the covalent attachment of dextran to STI but also demonstrated the broad molecular weight distribution of the conjugates. The STI-D conjugate retained satisfactory activity, although the attachment partially reduced its inhibitory activity against trypsin. The periodate oxidation method seemed to be the best for the preparation of STI-D since it gave the conjugate with a high modification ratio (4.6 molecules per STI), high yield (95%), and satisfactory activity recovery (63%). Chemical modification of STI was also carried out with activated polyethylene glycol (PEG) for comparison. The STI-PEG conjugate was obtained in a satisfactory yield (96%) and modification degree (5.8 molecules per STI), but the remaining activity was considerably lower (34%). Thus, conjugation of protein with dextran by the periodate oxidation method is suggested to be preferable for preparing a protein-carrier system without significant diminution of its biological activity.     

聚乙二醇化尿酸酶体内外稳定性研究

目的研究聚乙二醇化尿酸酶体内外稳定性。方法以酶活为指标,考察聚乙二醇修饰尿酸酶和尿酸酶的温度稳定性(4~80℃)、酸碱稳定性、抗胰蛋白酶水解能力和小鼠体内半衰期。结果4~60℃条件下,修饰的尿酸酶的稳定性大于尿酸酶,在70℃时,两者活性均迅速降低。pH 5.2~6.0及pH 9.2~10.0之间,尿酸酶活性迅速降低,而修饰的尿酸酶却保留了较高的活性。抗胰蛋白酶水解中,尿酸酶在作用200 min后,活性降至最高值的20%;而修饰的尿酸酶仍保留70%的活性。体内稳定性试验表明,修饰的尿酸酶和尿酸酶的半衰期分别为1 530和45 min。结论聚乙二醇修饰可以增加尿酸酶的稳定性和抗胰蛋白酶水解能力,延长体内半衰期。     

EXTENSIVE MODIFICATION OF PROTEIN AMINO GROUPS BY REDUCTIVE ADDITION OF DIFFERENT SIZED SUBSTITUENTS

The amino groups of ovomucoid, lysozyme and ovotransferrin have been extensively alkylated by reacting the proteins with various carbonyl reagents in the presence of sodium borohydride. The extent of modification ranged from 40 to 100%. Essentially monosubstitution was obtained with acetone, cyclopentanone, cyclohexanone and benzaldehyde, while 20–50% disubstitution was obtained with N-butanal and nearly 100% disubstitution was obtained with formaldehyde. Both the methylated and isopropylated derivatives of all three proteins were soluble and retained almost full biochemical activities, but introduction of the larger substituents caused precipitation with lysozyme and ovotransferrin.     

Effect of alkylation with different sized substituents on thermal stability of lysozyme

The amino groups of hen egg white lysozyme were reductively alkylated by the reaction with aliphatic aldehydes of various chain lengths and with two aldehydes of different steric hindrance at pH 7.5 and 4° for 3 h. About four of the original six lysine residues were modified by the reaction with acetaldehyde, n-butylaldehyde or n-hexylaldehyde. About three lysine residues were 2, 2-dimethylpropylated with trimethy-lacetaldehyde while a single residue was modified with benzaldehyde. The thermal stabilities of these alkylated lysozymes were investigated by differential scanning calorimetry (DSC) at different acidic pH values. Alkylation thermally destabilized the proteins, depending not only on the extent of modification but also on the size of the substituent. The alkylated derivatives were 8–19kJ/mol less stable than native lysozyme at 25° and pH 3.0. The temperature dependences of the activities of the alkylated lysozymes against ethylene glycol chitin indicated that the orders of the optimum temperatures and the maximum activities were exactly the same as the order of the thermal stabilities.     

A site‐specific PEGylated analog of exendin‐4 with improved pharmacokinetics and pharmacodynamics in vivo

Objectives Our aim was to improve the in vivo pharmacokinetics and pharmacodynamics of exendin‐4 by using site‐specific PEGylation. Methods We designed the PEGylated peptide based on its structure and activity relationship and prepared the conjugate by two steps of chromatographic purification. After obtained the conjugate we confirmed its glucose‐lowering activity in normal mice and determined its half‐life in SD rats. Then we evaluated its anti‐diabetic activity in a multiple low‐dose Streptozocin (STZ)‐induced diabetic mice model. Key findings With the process established in this study the product conjugate was obtained with a yield of over 60% and purity of above 99%. The conjugate maintained its original conformation after modification. In SD rats its half‐life was prolonged to 27.12 ± 5.75 h which was 17.61‐fold longer than that of the natural exendin‐4 for which the half‐life was only 1.54 ± 0.47 h. Its anti‐diabetic activity was significantly improved in the diabetic mice. Conclusions Compare with native exendin‐4, the C‐terminal site‐specific PEGylated analog of exendin‐4 obtained in this study has an improved pharmacokinetics and pharmacodynamics in vivo and could be regarded as a potential candidate for the future development of anti‐diabetic drugs.     

Novel Bioadhesive Chitosan-EDTA Conjugate Protects Leucine Enkephalin from Degradation by Aminopeptidase N

Purpose. To develop a novel bioadhesive polymer that protects peptide drugs from luminal degradation by aminopeptidase N and to evaluate the system in vitro on porcine mucosa. Methods. EDTA was covalently bound to chitosan in order to combine the bioadhesive properties of the polymer with the well known capacity of EDTA to complexe metal ions which are essential for the enzymatic activity of proteases. The inhibitory effect of this polymer conjugate was evaluated by using leucine enkephalin (Leu enkephalin) as a model drug. The degree of Leu enkephalin degradation caused by aminopeptidase N (EC 3.4.11.2), as well as porcine mucosa, in the presence of the polymer conjugate, was quantified by HPLC analysis. Results. The chitosan-EDTA conjugate is capable of binding 2.01 ± 0.12 mmole of zinc per gram of polymer at pH 6.5 (n = 3; ± S.D.). As zinc is an essential co-factor for aminopeptidase N, enzyme activity (48 mU/ml) could be completely inhibited under the use of 1.0% chitosan-EDTA conjugate. The inhibitory effect of 1.0% chitosan-EDTA conjugate on the degradation of Leu enkephalin on porcine mucosa within 3 h at 37°C was even 2.9-fold higher than that of a recently developed zinc complexing bacitracin-poly(acrylic acid) conjugate of the same concentration. The novel polymer conjugate is more bioadhesive than unmodified chitosan and is easily hydratable in water and basic aqueous solutions exhibiting quick swelling properties. Conclusions. The bioadhesive polymer conjugate described here seems to be a useful tool in overcoming enzymatic degradation by aminopeptidase N.     

Targeting of Doxorubicin to the Urinary Bladder of the Rat Shows Increased Cytotoxicity in the Bladder Urine Combined With An Absence of Renal Toxicity

Targeting of anti-tumor drugs to the urinary bladder for the treatment of bladder carcinoma may be useful, since these agents generally have a low degree of urinary excretion and are highly toxic elsewhere in the body. The anti-tumor drug doxorubicin was coupled to the low-molecular weight protein lysozyme via the acid-sensitive cis -aconityl linker. All free amino groups of the lysozyme were used for drug attachment to achieve intact excretion of the doxorubicin-aconityl-lysozyme conjugate into the bladder. In the bladder, the cytotoxic drug should be regenerated through acidification of the urine. First, the doxorubicin-aconityl-lysozyme conjugate was tested in rats for its target specificity and general toxicity. Wistar rats were injected intravenously with 2 mg/kg free doxorubicin or 10 mg/kg lysozyme-conjugated doxorubicin. Total urinary excretion of doxorubicin was about 10 times higher if the drug was coupled to lysozyme (39 ± 3% versus 4.4 ± 0.4%) . Free doxorubicin had no detectable toxic effects on heart, liver and lung but caused severe renal damage (proteinuria, N -acetyl-glucosaminidase excretion and glomerulosclerosis). None of the rats injected with doxorubicin-lysozyme conjugate showed such renal toxicity. Second, we tested whether doxorubicin could be released from the conjugate in the bladder through acidification of the urine and if the released doxorubicin could still exert a cytotoxic effect. Doxorubicin-aconityl-lysozyme (2 mg/kg conjugated doxorubicin, i.v.) was administered in rats with acidified urine (pH 6.1 ± 0.1) and in rats with a high urinary pH (8.2 ± 0.4). Ten times more doxorubicin was released from the conjugate in the group with acidified urine (15 ± 7% versus 1.7 ± 0.1%) . In agreement with this, cytotoxicity was also higher in the low pH group (IC 50 of 255 ± 47 nM versus 684 ± 84 nM doxorubicin). In conclusion, a specific delivery of doxorubicin to the urinary bladder combined with a reduced toxicity of doxorubicin in the kidneys can be achieved by coupling this anti-tumor drug to the low-molecular weight protein lysozyme via an acid-labile linker. A release of cytotoxic doxorubicin in the urinary bladder can be achieved by acidification of the urine. This technology, after further optimization, may provide an interesting tool for the treatment of bladder carcinoma.     

肿瘤微环境双重响应性的智能型蛋白毒素给药系统的构建和表征

目的 利用蛋白重组技术和PEG定点修饰技术,制备具有肿瘤微环境双重响应性的智能型蛋白毒素给药系统。方法 利用基因重组技术,在天花粉蛋白(trichosanthin,TCS)的C端引入天冬酰胺内肽酶(legumain)的底物天冬酰胺肽段和半胱氨酸残基,将所构建的突变体转化到大肠杆菌中表达目的蛋白并纯化。进一步将重组蛋白末端的半胱氨酸与具有巯基反应性的mPEG-Hz-Mal偶联合成TCS-Asn10-Hz-PEG,采用弱阳离子交换柱纯化TCS-Asn10-Hz-PEG,并在体外考察了TCS-Asn10-Hz-PEG的酸敏感性和酶敏感性。结果 成功制备、分离和纯化得到了TCS-Asn10-Cys,完成了mPEG-Hz-Mal与TCS-Asn10-Cys的定点偶联,得到智能型蛋白毒素给药系统TCS-Asn10-Hz-PEG。TCS-Asn10-Hz-PEG在体外pH 5.6的介质中和天冬酰胺内肽酶的作用下,能够水解或酶解释放出TCS,具有酸敏感特性和酶敏感特性。结论 本实验设计的蛋白毒素给药系统,具有酸敏感和酶敏感双重响应特性。     

Detection of human neutrophil elastase with peptide‐bound cross‐linked ethoxylate acrylate resin analogs

Abstract: An assessment of elastase‐substrate kinetics and adsorption at the solid–liquid interface of peptide‐bound resin was made in an approach to the solid‐phase detection of human neutrophil elastase (HNE), which is found in high concentration in chronic wound fluid. N‐succinyl‐alanine‐alanine‐proline‐valine‐p‐nitroanilide (suc‐Ala‐Ala‐Pro‐Val‐pNA), a chromogenic HNE substrate, was attached to glycine‐cross‐linked ethoxylate acrylate resins (Gly‐CLEAR) by a carbodiimide reaction. To assess the enzyme‐substrate reaction in a two‐phase system, the kinetic profile of resin‐bound peptide substrate hydrolysis by HNE was obtained. A glycine and di‐glycine spacer was placed between the resin polymer and substrate to assess the steric and spatial requirements of resin to substrate with enzyme hydrolysis. The enzymatic activities of suc‐Ala‐Ala‐Pro‐Val‐pNA and suc‐Ala‐Ala‐Pro‐Ala‐pNA on the solid‐phase resin were compared with similar analogs in solution. An increase in visible wavelength absorbance was observed with increasing amounts of substrate‐resin and enzyme concentration. Enzyme hydrolysis of the resin‐bound substrate was also demonstrated on a polypropylene surface, which was employed for visible absorbance of released chromophore. A soluble active substrate analog was released from the resin through saponification of the ethoxylate ester linkages in the resin polymer. The resin‐released conjugate of the HNE substrate demonstrated an increased dose response with increasing enzyme concentration. The synthesis and assay of elastase substrates bound to CLEAR resin gives an understanding of substrate‐elastase adsorption and activity at the resin's solid–liquid interface for HNE detection with a solid‐phase peptide.     

Isosteric conversion of protein carboxyl groups into carboxamide groups. II. Application to lysozyme

For isosteric conversion of carboxyl groups of proteins into amide groups, ammonolysis of protein esters under mild conditions was attempted. Ammonolysis of methyl esters of lysozyme and bovine serum albumin proved to be incomplete. Highly reactive N-ethylsalicylamide esters of guanylated lysozyme were therefore prepared by subjecting the protein to reaction with N-ethylbenz-isoxazolium ion at pH 4.2, 0°. Per molecule, 5–7 ester groups were introduced, with concomitant decrease of activity of 80–90%. Only 0.3 tyrosine was modified. On hydrolysis at pH 9.2 the activity was completely restored. At pH 7.9 three classes of ester groups could be distinguished: one group of high rate of hydrolysis (k₁ = 1.5 min^-1), three groups of intermediate rate (k₂ = 0.13 min^-1) and two groups of low rate (k³ = 0.018 min^-1). The intermediate rate approximated the rate of hydrolysis of the model compound benzoylglycine N-ethyl-salicylamide ester (k = 0.15 min^-1). Ammonolysis at pH 9.2 in 2.0 M ammonia/ ammonium acetate provided complete conversion of the ester groups into amide groups without restoration of activity, confirming the essentiality of certain carboxyl groups. In particular, rearrangement of the ester groups into relatively stable imide groups by O–N acyl migration was found to be completely absent. When native lysozyme was esterified with N-ethylbenzisoxazolium ion the activity did not completely return on hydrolysis.     

Histidine decarboxylase in the stomach of the rat

Two enzymes capable of decarboxylating l -histidine in vitro have been identified in rat and mouse stomach; one, located in the fundic portion, shows maximal activity at a pH value of 5.6, whilst the other, in the pyloric portion, requires a pH of 7.6 for optimal activity. The enzyme in rat fundus is stable when stored at low temperatures and is inhibited only slightly by benzene, α-methyldopa and α-methyl-histidine; the pyloric enzyme, on the other hand, is rapidly destroyed on storage at low temperatures, and is slightly stimulated by benzene but much inhibited by α-methyldopa and α-methylhistidine. Dopa and 5-hydroxytryptophan compete with the substrate for the pyloric histidine decarboxylase but have no effect on the fundic enzyme. Starvation inhibits the activity of the fundic enzyme but has only a slight effect on the pyloric enzyme. It is concluded that the two enzymes in the stomach capable of forming histamine differ from the specific and non-specific histidine decarboxylases found in other tissues.