Adenosine triphosphatase activity in early somite tissue of the chick embryo.

W E know from the work of Holtzer, Marshall, & Finck (1957), who used a fluorescent antibody staining technique, that by the end of the second day of development in the chick (stage 13, Hamburger & Hamilton, 1951) myosin begins to be identifiable in the myoblast cells of the somites. We have no information so far, however, about the distribution of adenosine triphosphatase in the somites at this early stage, and one of the most characteristic properties of myosin (Engelhardt, 1946) is that it dephosphorylates adenosine triphosphate (ATP), a process which occurs during muscular contraction. The only studies of ATP-ase activity in developing chick muscle (Moog, 1947; Robinson, 1952) have dealt with stages from 12 days' incubation onwards, and have used muscle not all of somitic origin. In an earlier publication Moog & Steinbach (1945) included data on stages as young as 3 days, but used the whole embryo for assays of the enzyme activity. Owing to a lack of adequate micromethods, it was not possible to estimate ATP-ase in somite tissue alone from stages as young as those in which Holtzer et al. found myosin antigen. In the following work a micromethod has been employed to measure ATP-ase activity in somite mesoderm of chick embryos at stage 11 (Hamburger & Hamilton) in which there are approximately equal quantities of segmented and unsegmented mesoderm (Text-fig. 1). The ATP-ase activity of the segmented tissue was compared quantitatively with that of the unsegmented tissue to see whether it was markedly higher in the segmented material, which would suggest that myosin was already being synthesized at this stage. In this study it was necessary to single out an ATP-ase activity that might be attributable to myosin, apart from the ATP-ase carried on the mitochondria of all cells. This was done partly by using different enzyme activators: mitochondrial ATP-ase is activated by magnesium ions, while the ATP-ase most typical of myosin is activated by calcium ions and inhibited by Mg. A differential centrifugation technique was also used to isolate the mitochondria. But the chief problem was, having detected