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M S Cohn

Publications and source records attributed to M S Cohn.

23 records · Page 2Linked to original sources
PubMed1977

Identification of a pyruvoyl residue in S-adenosylmethionine decarboxylase from Saccharomyces cerevisiae.

S-Adenosylmethionine decarboxylase from Saccharomyces cerevisiae has been purified to homogeneity. Acid hydrolysis of NaB3H4-reduced enzyme released 2.2 mol of tritiated lactate per mol of dimeric enzyme, indicating that a pyruvate moiety is present. Inhibition of enzymatic activity by NaBH4 reduction and by carbonyl-binding reagents indicates that this pyruvoyl residue is required for the activity of the enzyme. This is the first example reported of a eukaryotic enzyme containing a covalently linked pyruvoyl residue.

Adenosylmethionine Decarboxylase↗
PubMed1975

Catalytic and thermodynamic properties of the urocanate hydratase reaction.

Urocanate hydratase (4-imidazolone-5-propionate hydro-lyase, EC 4.2.1.49) isolated from Pseudomonas putida contains covalently bound alpha-ketobutyrate as its cofactor. In the process of examining the mechanism by which alpha-ketobutyrate serves in this capacity, various thermodynamic parameters and temperature effects on urocanate hydratase activity were determined. As the equilibrium constant at 15 degrees C for imidazooone propionate formation from urocanate is approximately 69, regardless of whether urocanic acid or chemically synthesized imidazolone propionate is used as the initial substrate, it is concluded that the reaction is freely reversible. DeltaG degrees ', deltaH degrees ' and deltaS degrees ' were --2.5 kcal/mole, +5.2 kcal/mole and +26 cal/deg mole, respectively. Measurement of first-order reaction rates at various temperatures, in order to calculate the Arrhenius activation energy, showed a sharp break in the Arrhenius plot at 29 degrees C. Further examination of this phenomenon by determining s20,w values of urocanate hydratase as a function of temperature revealed a dramatic change at 31 degrees C. Since the enzyme in both experiments reverts to its original state when the temperature is lowered back below the transition point, it is proposed that urocanate hydratase undergoes a reversible conformational change or partial dissociation which affects its catalytic properties in the range of 29--31 degrees C.

Binding Sites↗
PubMed1974

Involvement of threonine dehydratase in biosynthesis of the alpha-ketobutyrate prosthetic group of urocanase.

Seventeen mutants of Pseudomonas putida that were unable to grow on threonine as nitrogen source owing to a lack of threonine dehydratase were isolated, and all were found to be unable to synthesize active urocanase. Spontaneous revertants selected for urocanase production concomitantly regained threonine dehydratase. Mutants that were unable to utilize urocanate as carbon source were also isolated, and these were defective in urocanase formation but were normal in threonine dehydratase levels. Since alpha-ketobutyrate is the prosthetic group for urocanase, these results are consistent with the proposal that threonine dehydratase is necessary for urocanase prosthetic group biosynthesis. However, the lack of urocanase activity in threonine dehydratase-negative mutants was shown not to be the result of reduced levels of endogenous free alpha-ketobutyrate, nor to the participation of threonine dehydratase in the initiation of urocanase biosynthesis through the conversion of threonyl-tRNA(Thr) to alpha-ketobutyryl-tRNA(Thr). Other alternatives for the participation of threonine dehydratase in urocanase biosynthesis are discussed.

Butyrates↗
PubMedDate not supplied

Goretex and autogenous vein in the canine model.

Thirty Goretex grafts of 6 mm internal diameter, 30 mu fibril length, 30 mg/cc density and 0.5 mm wall thickness were placed as end-to-side bypass grafts in canine femoral arteries. Handling characteristics, patency and healing properties were compared with autogenous jugular veins which were anastomosed into the opposing femoral arteries. Autogenous veins were easier to anastomose and they showed a patency rate of 87% over a 16 week observation period. The Goretex grafts had a 50% patency rate during the same period. Goretex grafts failed because of technical errors (9), graft kinks (5), infection (3) and neointimal fibrous hyperplasia (2). This canine model has previously been used to evaluate all currently available biologic and synthetic vascular grafts. Goretex grafts demonstrated the highest rate of technical failures, but the lowest incidence of neointimal fibrous hyperplasia when compared to other grafts previously tested in this model.

Animals↗