| Summary: | 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.
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