Stimulation of Sphingomyelin Synthetase by Sulfhydryl Reagents

Abstract

Chicken liver mitochondrial sphingomyelin synthetase (CDPcholine:ceramide cholinephosphotrans-ferase, EC 2.7.8.3) was shown to be able to utilize erythro- as well as threo-ceramides as substrates for sphingomyelin synthesis; the former was incorporated only if sulfhydryl reagents were present in the incubation mixture. It was found that the most potent activator was menadione, while N-ethylmaleimide, oxidized glutathione, oxidized CoA, and CoQ0 were active, but to a much lesser extent. In contrast to the chicken liver synthetase, which showed broad specificity for sulfhydryl reagents, the synthetase from rat liver was only activated by oxidized CoA. The mechanism of activation is considered to occur as a result of a conformational change in the enzyme, subsequent to sulfhydryl alkylation, such that it allows the erythro- as well as threo-ceramide to penetrate to the active site.