Intracellular compartmentation of isozymes of sugar phosphate metabolism in green leaves.

The present paper has summarized evidence for the presence of two isozymes for many enzyme activities of sugar phosphate metabolism in plant leaves. These two isozymes are clearly compartmentalized in the chloroplasts and in the cytosol of plant leaf cells. In C4 plants there exists an additional isozyme in the mesophyll cells of these leaves in addition to the two isozymes in the bundle sheath cells. Such cell-compartment-specific and cell-specific isozymes provide duplicate (and possibly triplicate) enzyme systems for complete or almost complete pathways (ie, glycolysis, gluconeogenesis, and the oxidative pentose phosphate pathway). They provide a basis for the understanding as to how many isozymes one may expect in plants. They also provide a challenge to determine what their function is particularly in the differential regulation of metabolic pathways in different cell compartments. Based on his genetic analyses Weeden [1981] has recently proposed a model for the evolution of chloroplast-specific isozymes of sugar phosphate metabolism. This model rests on the endosymbiotic theory for the origin of chloroplasts. It still is highly speculative. However, cell-compartment specific isozymes may eventually provide a means of studying plant evolution, especially if we succeed in analyzing their primary structure.