Specificity of protein turnover in tomato leaves. Accumulation of proteinase inhibitors, induced with the wound hormone, PIIF.

Detached tomato leaves, supplied with the proteinase inhibitor inducing factor (PIIF) and incubated with water under constant light, exhibited a specificity of intracellular protein turnover directed toward the selective accumulation of heat-stable proteins having disulfide corss-linkages. Approximately 70% of the accumulated proteins could be accounted for in two proteinase inhibitors rich in disulfide links. The accumulation of proteins containing disulfides was accompanied by a net loss in total leaf protein, mainly of heat-precipitable proteins having free sulfhydryl residues. Relative rates of synthesis of --S--S-- proteins and --SH proteins were assessed by comparing rates of incorporation of isotope into the inhibitor proteins and noninhibitor leaf proteins. Although the inhibitors represented about 12% of total leaf protein after 71 h of induction, only about 2% of total protein synthesis was directed toward inhibitor synthesis during incubation of induced leaves. The marked stability of inhibitors, and other disulfide proteins against degradation in vivo, appeared to be a major factor providing for their selective accumulation. It was concluded that the state of oxidation of protein-bound half-cystine residues may be a principle parameter influencing the susceptibility of leaf proteins to degradation in vivo.