In vitro and in vivo regulation of chJoroplast glyceraldehyde‐3‐phosphate dehydrogenase isozymes from Chenopodium rubrum. III. The molecular basis of the aggregation phenomenon: chloroplast glyceraldehyde‐3‐phosphate dehydrogenase as an ambiquitous enzyme

The nature of the aggregated form of chloroplast glyceraldehyde‐3‐phosphate dehydrogenase isozymes (GPD, EC 1.2.1.13) from Chenopodium rubrum leaves was investigated. After disaggregation of the isozymes in NADP + buffer, and resuspension of the disaggregated isozymes in NAD + buffer, complete reaggregation could only be achieved by remixing the enzyme with a high molecular weight fraction, from which the isozymes had dissociated during the NADP + filtration. After separation of the isozymes by inverse ammonium sulphate gradient solubilization, spontaneous extensive reaggregation of each isozyme was observed in NAD + buffer. The high molecular weight material consisted of ribonucleoprotein, and RNase treatment impaired its ability to promote reaggregation of chloroplast GPD. It is proposed that pyridine nucleotide‐controlled aggregation and binding to ribonucleoprotein in vitro are artifacts which reflect an in situ binding to cellular components. Since uncontrolled NAD + ‐linked activities of the bifunctional isozymes in the chloroplast would lead to an equalization of the NAD + and NADP + redox couples, it is suggested that the reversible binding of the isozymes forms the basis of a regulatory system in vivo.