Close resemblance between muscle pyruvate kinase from a primitive vertebrate, the river sturgeonAcipenser fulvenscens, and the ancestral type K isozyme

Summary

Pyruvate kinase (PK) exists as at least two electrophoretically distinguishable, tissue specific form in the river sturgeonAcipenser fulvenscens. In contrast with the PK isozyme system in other vertebrates, the isozyme present in muscle is also present in non-musclce tissues, notably the spleen. The presence of a five membered set of hybrids in liver, gills, eyes and intestine suggests that the synthesis of this isozyme occurs in other tissues as well. Thus, in this primitive vertebrate, the specialization of one PK isozyme for function in muscle tissues is not complete. Sturgeon muscle PK had slightly cooperative phosphoenolpyruvate (PEP) saturation curves (S_0.5=0.33 mM) and a Hill coefficient,n _H, of 1.24, and was strongly activated by fructose 1,6 bisphosphate (PEP S_0.5=0.098 mM;n _H=1.0). The enzyme was strongly inhibited by phenylalanine, alanine and MgATP. Fructose 1,6 bisphosphate (FBP) not only totally reversed these inhibitory effects, but also activated the enzyme to the same extent as in the absence of the inhibitors. These kinetic properties closely resemble those of vertebrate type K pyruvate kinases, and are quite distinct from those of higher vertebrate type M isozymes. Nevertheless, immunotitration studies indicate structural homology among the sturgeon, frog and dog muscle PK's as they are all precipitated by antisera to the other forms. This maintenance of structural homology, despite a marked degree of functional differentiation, suggests that only small structural modifications were required to generate the functionally specialized forms of muscle PK found among the vertebrates.