Cysteine Metabolism in Cultured Tobacco Cells

Abstract

Transported l-[³⁵S]cysteine was rapidly metabolized by cultured tobacco cells when supplied to the cells at 0.02 millimolar or 0.5 millimolar. The internal cysteine pool was expandable to approximately 2400 nmoles per gram fresh weight.

The ³⁵S label derived from cysteine was found in several metabolites. The amount of label in glutathione and sulfate was directly proportional to the internal l-[³⁵S]cysteine, while the levels of labeled methionine and protein were apparently independent of internal labeled cysteine. Cysteine was more rapidly metabolized when the external cysteine concentration was low (0.02 millimolar) with up to 90% of the ³⁵S label present as compounds other than cysteine.

The initial step in cysteine degradation yielded pyruvate, sulfide, and presumably NH₄⁺. Stoichiometry studies using extracts prepared from acetone powders of tobacco cells indicated that pyruvate and sulfide were produced in a 1:1 ratio. The catabolic reaction was linear with respect to time and amount of protein and had a pH optimum of 8 in crude extracts. Preliminary kinetic data indicated the K  _m to be approximately 0.2 millimolar. The extractable degradative activity was enhanced 15- to 20-fold by preincubating the cells for 24 hours in 0.5 millimolar cysteine. The extractable specific enzyme activity roughly reflected the growth curve of the cells in culture. Maximal cysteine degradation was observed in extracts prepared from late log phase cultures that were preincubated in cysteine, while little activity was found in similar extracts from stationary phase cultures. These results are consistent with an inducible catabolic enzyme similar to the cysteine desulfhydrase from bacteria.

1

Present address: Comparative Animal Research Laboratory, 1299 Bethel Valley Road, Oak Ridge, Tennessee 37830.