Oxidative phosphorylation in whole homogenates and in cell particles.

While many current investigations involving oxidative phosphorylation are being carried out either with whole homogenates or with crude fractions derived therefrom, there are few in which (a) the results have been reported relative to the activity of the original homogenate, and (b) the fractions have been tested in various combinations. The need for such comparisons was shown in earlier results from this laboratory (1, 2). Although rat liver mitochondria contained only 25 per cent of the nitrogen in the whole homogenate, they contained 45 per cent of the “oxalacetic acid oxidase” activity. The remaining fractions when tested alone contained al. most no activity, but when they were added to the mitochondria the activity of the whole homogenate was regained. It thus appears that the rate-limiting reaction in the mitochondrial preparation was not to be identified with the rate-limiting reaction in the whole homogenate or in the remaining fractions, and in none of the combinations can the rate-limiting reaction be said to be positively identified. The complex of reactions that comprise oxidative phosphorylation represents a delicate balance between phosphate breakdown and phosphate uptake, and the enzymes involved in phosphate breakdown could conceivably be present in the nonmitochondrial fractions of the cell at levels sufficient to “stimulate” the mitochondria to a higher level of oxidative activity. One approach to this problem is to assay t.he whole homogenate and the fractions for individual enzymatic components (3-6). Another is to study the balance between phosphate uptake and phosphate breakdown in the respective combinations. The latter method was employed in the present investigation.