Incorporation of Cystine by Yeast Cells of Histoplasma capsulatum

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This report deals with the incorporation of cystine into macromolecules by yeast cells of Histoplasma capsulatum. The results show that at saturating concentrations of cystine in a rich medium, total uptake of the cystine occurs in 10 hr, whereas the amount of label in the cold trichloroacetic acid-soluble material reaches a maximum at 3 to 4 hr and remains at this value. The amount of label in the cold acid-insoluble material accumulates linearly for 4 to 5 hr and reaches a plateau at 7 to 9 hr. A chemical fractionation of labeled cells shows that 25% of the cystine taken up remains as low-molecular-weight components, of which cystine comprises 60 to 75%. Approximately 30% of the total label incorporated is ethanol-soluble, and polyacrylamide gel electrophoresis of this material shows a rather uniform incorporation of the amino acid into many proteins. The combined hot KOH fractions account for 40% of the total label incorporated. The amount of hot KOH-insoluble material almost doubles in a 10-hr pulse, whereas there is an increase in hot KOH-soluble material. Hence, the greatest amount of label from cells pulsed with 14C-cystine is recovered from cell wall material.

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