The assay of animal tissues for respiratory enzymes; the malic dehydrogenas system.

The “homogenate technique” is at present widely used for the study of enzymatic reactions in various types of biological material. In the original technique (1)’ an attempt n-as made to eliminat,e cell structure and to disperse intracellular constitucnt,s so widely that endogenous respiration was eliminated by dilution, while particular phases of metabolism could be restored by the addition of appropriate substrates and cofact.ors. In 1942, Elliott and Libet (5) made an excellent study which led to t,he conclusion that’ “the effects of homogenization in the different media are mainly due to the tendency of isotonic salts or sugars to prevent cell cytolysis, while in hypotonio medium cytolysis occurs.” They reported that, isotonic preparations of brain cells compared favorably with brain slices for studies on brain metabolism, and they subsequently (6) carried out their st,udies with such preparations unfortified with coenzymes or with cytochrome. On the other hand, Schneider and Potter (7, 8) pursued the alternative which Elliott and Libet’s work suggested and homogenized tissues in dist.illed water in order to make cytolysis as complete as possible. We propose to refer to this type of preparation as a “water homogenate,” while that of Elliot,t et al. (6) may be referred to as an “isot,onic hom0genat.e.” On t,he basis of the cytorhrome c content of liver and t.he amount of cytochrome required to saturat,e the succinoxidase system (9) an objective technique for estimating the degree of cytolysis was devised (7, 8). To date no other method of obt.aining a quantit,ative measure of cytolysis has been found; microscopir exa.minations cannot give quantitat,ive data and in fact can give very erroneous impressions. According to the succinoxidase t,est , wtt,er homogenates of rat liver contain about, 5 per cent whole cells, while isot,onic preparations contain 60 to 80 per cent whole cells. As might be