Dissimilar properties of pyruvate kinase from rabbit and hare muscle

Some catalytic and kinetic properties of pyruvate kinase (PK, EC 2.7.1.40) isolated from rabbit and hare heart and skeletal muscle with a 9-16-fold degree of purification were studied. The initial specific activity of the enzyme in hare heart homogenates was by 66% while in hare skeletal muscle by 25% higher than in respective rabbit tissues. Temperature optimums and thermostability of PK from hare tissues were higher as compared with those in rabbits. A comparison of K (M) (S-0.5) values shows that PK from hare skeletal muscle exhibits a highest affinity to phosphoenol pyruvate, but lowest to ADP, as compared with PK from rabbit skeletal muscle. Moreover, PK from both hare tissues exhibits a positive kinetic cooperativity (Hill coefficient > 1.35) of the phosphoenol pyruvate and ADP binding sites. In contrast to PK from rabbit tissues, the enzyme from hare heart and skeletal muscle is more sensitive to the inhibiting effect of ATP. It is suggested that in hare skeletal muscle PK is presented by its allosteric isoform which might be advantageous under extreme conditions of the hare's habitation.