On the metabolism of sulphur in excised roots

Abstract

The assimilation of several sulphur compounds in excised roots grown in culture was studied. To obtain sulphate deficient conditions a sulphate impurity was removed from the sugar component of the medium by ion exchange. The sulphur compounds investigated were sulphate, taurine, cysteic acid, elemental sulphur, cystine, cystamine, glutathione, homocystine and methionine. Of these possible sources of sulphur, sulphate, cystine, homocystine and methionine were utilized for growth. All four were available for the synthesis of protein cysteine and methionine. The roots responded to elemental sulphur but whether the roots assimilated the sulphur before or after oxidation to sulphate was not established. 2. The radioactive sulphur compounds present in tomato root extracts were separated after ion exchange fractionation by thin layer electrophoresis and chromatography. Thiols present in these extracts were protected from oxidation by reaction with iodoacetamide, but acetamidocysteine was later found to be unstable. 3. Among the compounds labelled by incubating tomato roots for quarter of an hour with carrier-free (³⁵S)sulphate, were glutathione, cysteine and methionine as well as protein cysteine and methionine. 4. When both sulphate and methionine were supplied to roots, exogenous methionine was incorporated preferentially into protein methionine whereas the label of sulphate, after assimilation into cysteine was incorporated preferentially into protein cysteine. Glutathione was but slightly labelled and cysteine not at all by methionine in the presence of sulphate so sulphate or a metabolite reduced the conversion of methionine into cysteine and supplied most of the sulphur for cysteine synthesis.