The enzymic incorporation of choline-l,2-'C from CDP-choline-1 ,V4C into phosphatidylcholine by spinach leaf preparations was characterized. The enzyme catalyzing the incorporation, choline phosphotransferase, had a pH optimum of about 8.0 and required either Mn2+ or Mg2+ as cofactor. The saturation concentration of Mn2+ was 0.3 mM and that for Mg2+ was 13 mM. The nT, for CDP-choline was 10 p ~...

At least four different proteinases are present in senescing apple leaves (Malus domestica Borkh. cv. Golden Delicious) as determined by their pH optima, substrate specificity, and their reactivity to proteinase inhibitors. An enzyme active at pH 4.5 to 5.0 appears to be a sulfhydryl-dependent (iodoacetamide and phenylmercuric acetate-sensitive) endoproteinase, and degradation of the large subu...

Bacillus subtilis glutamine synthetase readily reacts with iodoacetamide, producing very specific changes in the catalytic parameters characteristic of the native enzyme. The alkylation reaction is influenced both by substrates and feedback inhibitors of the enzyme; whereas ammonia and glutamate are without effect, Mg2+-ATP enhances the alkylation reaction, and Mn2f-ATP effectively prevents the...

A major protease present in dormant bark tissues of the apple (Malus domestica Borkh. cv. "Golden Delicious") was partially purified by hemoglobin-coupled Sepharose column chromatography. The protease active at pH 4.6 and at temperatures of 30 to 50 C was found to be sulfhydryl-dependent. Phenylmercuric acetate inhibited the enzyme approximately 50% at 2 millimolar, whereas phenylmethylsulfonyl...

The enzymic incorporation of choline-1,2-(14)C from CDP-choline-1,2-(14)C into phosphatidylcholine by spinach leaf preparations was characterized. The enzyme catalyzing the incorporation, choline phosphotransferase, had a pH optimum of about 8.0 and required either Mn(2+) or Mg(2+) as cofactor. The saturation concentration of Mn(2+) was 0.3 mm and that for Mg(2+) was 13 mm. The K(m) for CDP-cho...

A factor catalyzing the in vitro degradation of oat phytochrome in crude extracts has been shown to be a proteolytic enzyme. The enzyme, an endoprotease, has been purified about 600-fold from dark-grown oat shoots by chromatography on ion exchange and molecular seive gels. The pH-activity curve is broad, with a maximum around pH 6.4. The enzyme is apparently dependent on the presence of reduced...

DPN-linked isocitrate dehydrogenase from bovine heart contains 6 half-cystine residues per subunit of molecular weight of 42,000. All of these residues are present as cysteine since six sulfhydryl groups per subunit can be modified by 5,5’-dithiobis(2-nitrobenzoate) (DTNB). Spectrophotometric measurements indicate that the modilication of three thiol groups which react preferentially with DTNB ...

DPN-linked isocitrate dehydrogenase from bovine heart contains 6 half-cystine residues per subunit of molecular weight of 42,000. All of these residues are present as cysteine since six sulfhydryl groups per subunit can be modified by 5,5’-dithiobis(2-nitrobenzoate) (DTNB). Spectrophotometric measurements indicate that the modilication of three thiol groups which react preferentially with DTNB ...

In recent years a great deal of evidence has accumulated to show that intact -SH groups are necessary for the full activity of a variety of enzymes. The exact function of the thiol group in the protein part of these “sulfhydryl enzymes,” however, remains obscure. It has been suggested that the -SH group directly participates in the activity of these enzymes, and that it might be actually locate...