Michael Bárány

Myosin was isolated from 14 different muscles (mammals, lower vertebrates, and invertebrates) of known maximal speed of shortening. These myosin preparations were homogeneous in the analytical ultracentrifuge or, in a few cases, showed, in addition to the main myosin peak, part of the myosin in aggregated form. Actin- and Ca(++)-activated ATPase activities of the myosins were generally proportional to the speed of shortening of their respective muscles; i.e. the greater the intrinsic speed, the higher the ATPase activity. This relation was found when the speed of shortening ranged from 0.1 to 24 muscle lengths/sec. The temperature coefficient of the Ca(++)-activated myosin ATPase was the same as that of the speed of shortening, Q(10) about 2. Higher Q(10) values were found for the actin-activated myosin ATPase, especially below 10 degrees C. By using myofibrils instead of reconstituted actomyosin, Q(10) values close to 2 could be obtained for the Mg(++)-activated myofibrillar ATPase at ionic strength of 0.014. In another series of experiments, myosin was isolated from 11 different muscles of known isometric twitch contraction time. The ATPase activity of these myosins was inversely proportional to the contraction time of the muscles. These results suggest a role for the ATPase activity of myosin in determining the speed of muscle contraction. In contrast to the ATPase activity of myosin, which varied according to the speed of contraction, the F-actin-binding ability of myosin from various muscles was rather constant.

31P nuclear magnetic resonance spectra recorded from intact muophosphate, and the sugar phosphates. Quantitation of these metabolites by 31P nuclear magnetic resonance was in good agreement with values obtained by chemical analyses. The spectra obtained from various muscles showed considerable variation in their phosphorus profile. Thus, differences could be detected between (a) normal and diseased muscle; (b) vertebrates and invertebrates; (c) different species of the same animal. The time course of change in phosphate metabolites in frog muscle showed that ATP level remains unchanged until phosphocreatine is nearly depleted. Comparative studies revealed that under anaerobic conditions the Northern frog maintains its ATP content for 7 hours, while other types of amphibian, bird, and mammalian muscles begin to show an appreciable decay in ATP after 2 hours. Several lines of evidence indicated that ATP forms a complex with magnesium in the muscle water: (a) the phosphate resonances of ATP in the muscle were shifted downfield as compared to those in the alkaline earth metal-free perchloric acid extract of the muscle; (b) the coupling constants of ATP measured in various live muscles closely corresponded to those for MgATP in a solution resembling the composition of the muscle water; (c) in the muscle the gamma-phosphate group of ATP exhibited no shift change over a period of 10 hours under conditions where resonances of other phosphate compounds could be titrated. This behavior is similar to that of MgATP in model solutions in the physiological pH range, and it is different from that of CaATP. The chemical shifts of the phosphate metabolites were determined in several relevant solutions as a function of pH. Under all conditions only inorganic orthophosphate showed an invariant titration curve. From the chemical shift of inorganic phosphate observed during aging of intact muscle the intracellular pH of frog muscle was estimated to be 7.2.

1. The characteristics of isometric twitch and tetanic contractions have been determined for normal (N-EDL, N-SOL), self-innervated (S-EDL, S-SOL) and cross-innervated (X-EDL, X-SOL) extensor digitorum longus (EDL) and soleus (SOL) muscles of the rat at 35 degrees C. The muscles were then used for biochemical analyses of properties of myosin and actomyosin.2. The ATPase activities of myosin and actomyosin of X-EDL decreased to the level of those of N-SOL or S-SOL, and the ATPase activities of X-SOL approached those of N-EDL or S-EDL. Of the various ATPase activities, the actin- and Mg(2+)-activated ATPase activity of myosin and the Mg(2+)-activated ATPase activity of actomyosin showed the highest degree of correlation with the intrinsic speed of shortening of the muscles.3. Myosin of normal, self-innervated, and cross-innervated muscles combined with F-actin superprecipitated at rates which were proportional to the speed of muscle contraction.4. The pH profile curve and the ATP-induced dinitrophenylation reaction revealed that the structure of myosin of X-EDL was altered to that of N-SOL or S-SOL, and the structure of myosin of X-SOL was modified to that of N-EDL or S-EDL.5. No differences were found in the yield of myosin of normal, self-innervated, and cross-innervated extensor digitorum longus and soleus muscles.