1. The presence of S-adenosylmethionine decarboxylase in human prostate gland is reported. A satisfactory radiochemical enzymic assay was developed and the enzyme was partially characterized. 2. Putrescine stimulates the reaction rate by up to 6-fold at pH7.5: the apparent activation constant was estimated to be 0.13mm. The stimulation is pH-dependent and a maximal effect is observed at acid pH values. 3. Putrescine activation is rather specific: other polyamines, such as spermidine and spermine, did not show any appreciable effect. 4. The apparent K(m) for the substrate is 4x10(-5)m. The calculated S-adenosylmethionine content of human prostate (0.18mumol/g wet wt. of tissue) demonstrates that the cellular amounts of sulphonium compound are saturating with respect to the enzyme. 5. The enzyme is moderately stable at 0 degrees C and is rapidly inactivated at 40 degrees C. The optimum pH is about 7.5, with one-half of the maximal activity occurring at pH6.6. 6. Several carboxy-(14)C-labelled analogues and derivatives of S-adenosylmethionine were tested as substrates. The enzyme appears to be highly specific: the replacement of the 6'-amino group of the sulphonium compound alone results in a complete loss of activity. 7. Inhibition of the enzyme activity by several carbonyl reagents suggests an involvement of either pyridoxal phosphate or pyruvate in the catalytic process. 8. The inhibitory effect of thiol reagents indicates the presence of ;essential' thiol groups.
Arginase was purified about 1800-fold from extracts of human full-term placenta; the enzyme appeared to be homogenous by disc electrophoresis and molecular-sieve chromatography. The mol. wt. determination by gel filtration and sodium dodecyl sulphate/polyacrylamide-gel electrophoresis yielded a value of 70000 for the most pure and the partially purified enzyme. The human placenta arginase is a metalloenzyme with an optimum pH of 9.1. The Km for L-arginine is 27 mM. L-Ornithine and L-lysine show competitive inhibition with Ki values of 6.3 and 14 mM respectively.
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