A study of the catalytic properties of pyruvate kinase isozymes from salmon and an examination of their functional relationships

Abstract

AbstractTwo major isozymes of pyruvate kinase that differ in electrophoretic and kinetic properties are found in salmon (Oncorhynchus kisutch). Type M occurs in cardiac and skeletal muscle, while type K is the main isozyme of the other tissues. We have been examining the structural and kinetic properties of the pyruvate kinase isozymes of salmon in an effort to assess their regulatory properties as well as their structural and functional relationships to the isozymes from “higher” vertebrates. We find that type K pyruvate kinase has hyperbolic kinetics with both phosphoenolpyruvate and ADP but that the apparent affinities for the two substrates are interdependent. That is, decreased affinities for phosphoenolpyruvate occur at higher concentrations of ADP and vice versa. In contrast, salmon type M pyruvate kinase has slightly sigmoidal kinetics with phosphoenolpyruvate, indicative of cooperative interactions among subunits of the latter isozyme. However, the affinity of this isozyme for each substrate is much less effected by the concentration of the other substrate. Salmon type K pyruvate kinase binds Mg++ cooperatively in the absence of fructose bisphosphate, where as type M binds Mg++ twice as tightly but in a non cooperative fashion. Both enzymes are inhibited to a similar extent by ATP, but they may differ in the nature of this inhibition. The salmon isozymes differ from avian and mammalian pyruvate kinase in that they are much less susceptible to inhibition by alanine and phenylalanine. The kinetic properties of salmon type K pyruvate kinase imply it to be more amenable to regulation that salmon type M, a situation that is also true for the corresponding isozymes from birds and mammals. Nevertheless, the two salmon isozymes appear to be more similar kinetically to each other than are types K and M pyruvate kinases from higher vertebrates, suggesting less evolutionary divergence for the salmon isozymes.