Functional studies with the octameric and dimeric form of mitochondrial creatine kinase. Differential pH-dependent association of the two oligomeric forms with the inner mitochondrial membrane

Phosphate extraction of mitochondrial creatine kinase (Mi-CK, EC 2.7.3.2) from freshly isolated intact mitochondria of chicken cardiac muscle, after short swelling in hypotonic medium, yielded more than 90% of octameric and only small amounts of dimeric Mi-CK as judged by fast protein liquid chromatography-gel permeation analysis of the supernatants immediately after extraction of the enzyme. In extraction buffer, octameric Mi-CK displayed a tendency to dissociate, albeit at a slow rate with a half-life of approximately 3-5 days, into stable dimers. Experiments with purified Mi-CK octamers or dimers, or defined mixtures thereof, incubated under identical conditions with Mi-CK-depleted mitoplasts revealed that both oligomeric forms of Mi-CK can rebind to mitoplasts. However, the association of Mi-CK was strongly pH-dependent and, in addition, octameric and dimeric Mi-CK showed different pH dependences of rebinding. Therefore, it was possible under certain pH conditions to rebind either both oligomeric forms or selectively the octamers only. Furthermore, evidence is presented that Mi-CK dimers partially form octamers upon rebinding to the inner membrane. The differential association of the two oligomeric Mi-CK forms with the inner mitochondrial membrane together with the dynamic equilibrium between octameric and dimeric Mi-CK (Schlegel, J., Zurbriggen, B., Wegmann, G., Wyss, M., Eppenberger, H.M., and Wallimann, T. (1988) J. Biol. Chem., 263, 16942-16953) suggest that both oligomeric forms are physiologically relevant. A change in the octamer to dimer ratio may influence the association behavior of Mi-CK in general and thus modulate mitochondrial energy flux as discussed in the phosphoryl creatine circuit model (Wallimann, T., Schnyder, T., Schlegel, J., Wyss, M., Wegmann, G., Rossi, A.-M., Hemmer, W., Eppenberger, H.M., and Quest, A.F.G. (1989) Prog. Clin. Biol. Res. 315, 159-176.