Abstract
A protein has been studied in human fetal liver that is able to condense serine with several sulfur donors. The two activities of greatest interest are cystathionine β-synthase (EC 4.2.1.22) which catalyzes the reaction of sulfide with serine to form cysteine, and l-serine dehydratase (EC 4.2.1.13) which condenses homocysteine with serine to form cystathionine. These two activities were followed through several purification steps and were always found in the same fractions. The ratio of the activities remained constant at 1.7 ± 0.1 with homocysteine as the preferred sulfur donor. When the two activities were assayed simultaneously, sulfide did not interfere with cystathionine synthesis, but a sulfide-to-homocysteine ratio of at least 1.5 was needed for cysteine production. Both activities showed similar thermal denaturation profiles and sensitivity to hydroxalamine. Further evidence that these activities are associated with the same protein comes from the observation that a patient with homocystinuria is deficient in cystathionine β-synthase as well as l-serine dehydratase.
Published Version
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