THE MACHINE AS ENZYME AND THE FUEL AS SUBSTRATE

The discovery by Engelhardt & Lyubimova in 1939 of the ATPase activity of myosin opened a new era in muscle biochemistry. Lundsgaard (8) had suggested that breakdown of ATP might be associated with restoration of the contractile substance, and D. M. Needham (1) that possibly ATP had some special spatial relationship to the myosin micellae. But the idea of the enzymic activity of the muscle machinery itself was an entirely new one, and the Russian workers fully realised its implications. They endeavoured to free the myosin from enzymic activity by repeated washing and reprecipitation but instead the activity rose to a fairly constant level. The purified myosin split off only one phosphate group, yielding ADP. They remarked on the great heat-lability of the ATPase, its activity being lost in 10 min at 37° and compared this with the low coagulation temperature of myosin known since the time of Kühne; they also noticed the similar sensitivity to acids of myosin as protein and as ATPase. These similarities served to increase the probability of the identity of the ATPase and myosin, but Engelhardt & Lyubimova considered that no final decision could be taken.

In 1941 Engelhardt, Lyubimova & Meitina (1) were the first to test the effect of ATP on myosin threads. These, though containing only about 2 % of protein, showed a certain amount of tensile strength; they were immersed in fluid and connected with the lever of a torsion balance so that when tension (about 200 mg) was applied the extensibility could be measured. They found that addition of 5 × 10−3M ATP caused considerable increase, 50–100%, in the extensibility.