The Adaptation of Enzymes to Pressure II. A Comparison of Muscle Pyruvate Kinases from Surface and Midwater Fishes with the Homologous Enzyme from an Off-Shore Benthic Species

Abstract

SYNOPSIS. Pyruvate kinase (PK) maximum catalytic rate is dramatically decelerated by increasing hydrostatic pressure. In four different species inhabiting different portions of the water column, muscle pyruvate kinase displays (1) a reduction in the volume change of activation, ΔV*. a t higher temperatures, (2) a pH independence of ΔV*, and (3) a general increase in the activation energy at increased pressures. Although shared by the four different pyruvate kinases examined, none of these characteristics is critically involved in the regulation of PK catalysis. In contrast, pressure effects on another set of characteristics, all vitally important to control of PK catalytic function, depend upon the species origin of the enzyme. In the case of the rainbow trout, Salmo gairdneri, high pressure dramatically reduces pyruvate kinase affinities for the two substrates (PEP and ADP), the cationic cofactor (Mg²⁺), and the negative modulator (ATP). The homologous muscle enzyme from Oligoplites mundus, another surface dwelling species, displays similar responses to pressure. On the other hand, muscle PK affinities for the same key regulatory ligands are much less pressure sensitive in the abyssal rattail fishes (Coryphaenoides sp.) and are essentially pressure independent in a vertically migrating midwater sea bass, Ectreposebastes imus. In these latter two species, PK catalytic rates under probable physiological conditions are determined largely by these kinetic properties rather than by energy-volume parameters.