Effects of some phenoloxidase inhibitors on chloroplasts and carboxylating enzymes of sugar cane and spinach

Summary A number of additives have been tested for their effects on o-diphenol: O₂ oxidoreductase activity of cane leaves. The most inhibitory compounds were thioglycollate, -mercaptoethanol, polyethylene glycol and bovine serum albumin. Sulphydryl (SH) compounds did not affect rates of photosynthetic CO₂ assimilation when used at concentrations below 10^-2 M. However, in the presence of Mn⁺⁺ ions they contributed to an O₂ consumption which masked photosynthetic O₂ evolution. Addition of SH compounds or of polymers to the grinding media increased rates of enzymic CO₂ assimilation in crude enzyme preparations from cane leaves, but did not affect rates of CO₂ assimilation in similar spinach preparations. Strong reducing agents, copper chelators, low O₂ tension and high pH were effective in reducing phenoloxidase activity, but presented problems in the isolation and assay of chloroplasts. The results are discussed in relation to (a) design of suitable media for preparation of active cane chloroplasts and (b) comparative studies of enzyme levels in plants of various genera.