Inorganic sulphate, sulphite and sulphide as sulphur donors in the biosynthesis of sulphur amino acids in germ-free and conventional rats

Abstract

1. 1. Incorporation of 35S from 35S-labelled sodium sulphate, sulphite and sulphide into the sulphur-containing amino acids cysteine/cystine and methionine was investigated in young germ-free and conventional rats. The animals were killed 3 h after intraperitoneal injection of the labelled compound. 2. 2. After injection of [ 35S]sulphate or [ 35S]sulphite both cysteine and methionine were labelled slightly in conventional rats, whereas no labelling of sulphur amino acids was detectable in germ-free animals. 3. 3. After injection of [ 35S]sulphide, cysteine was labelled to approximately the same extent in both conventional and germ-free rats. Labelling of methionine in conventional animals was much smaller and in germ-free rats not detectable. 4. 4. These findings prove that inorganic sulphur at the sulphide level of oxidation can be used in rat tissues for the synthesis of cysteine by a mechanism not involving methionine as the ultimate sulphur donor, most likely by enzymic sulphhydrylation of serine. The complete lack of incorporation of 35C from labelled sulphate and sulphite into cysteine in germ-free animals indicates that these compounds are not reduced to sulphide in noteworthy amounts in rat tissues. The intestinal microorganisms are thus responsible for the utilization of sulphate and sulphite in the sulphur amino acid synthesis observed in contentional rats.