The effect of tonicity of the medium on the respiratory and phosphorylative activity of heart-muscle sarcosomes

Abstract

The alpha-ketoglutaric oxidase system of heart-muscle sarcosomes has a pH optimum at 7.4. The yield of oxidative phosphorylation (P:O ratio) is unchanged between pH 6.2 and 7.7. Hypertonic sucrose (0.88 [image] is an inhibitor of the succinic oxidase system in the Keilin and Hartee prepn. Its major effect appears to be on the accessibility of both the endogenous and added cytochrome c to the other components the system. Max. activity of the alpha-ketoglutaric oxidase system of heart-muscle sarcosomes is obtained under the most highly hypotonic conditions studied, equivalent to about 1/3 isotonic. Under these conditions sarcosomes are swollen, but shrink again when placed in isotonic medium. The effect of tonicity on the activity of the alpha-ketoglutaric oxidase system is also reversible. As the tonicity is increased by saline, sucrose or phosphate, the activity of the alpha-ketoglutaric oxidase system decreases. The P:O ratio is not affected over a wide range of sucrose concns. which have a marked effect on the activity of the alpha-ketoglutaric oxidase system. This and other examples where the oxidase system is more sensitive than the P:O ratio to variations of the conditions indicates that the phosphorylative enzymes are normally in excess of the purely oxidative enzymes and increases the likelihood that measurements of the yield of oxidative phosphorylation on isolated tissue prepns. represent the state of affairs in the cell. Phosphate, in high concn., decreases the P:O ratio; the optimal concn. is 0.03 [image]. Hypertonic sucrose is unsuitable for the isolation of sarcosomes. There is, however, no significant difference between sarcosomes isolated with isotonic sucrose and isotonic saline, except that the latter are deficient in cytochrome c.