A study on the properties of mitochondria from rat kidney cortex and red medulla

Mitochondria of rat kidney red medulla form a single band (rho = 1.163) on a sucrose gradient, while mitochondria of the cortex form bands in 2 density regions, namely ca. 66% (M1) at rho = 1.173 and ca. 33% (M2) at rho = 1.163. The mitochondria of the red medulla contain more cytochrome a, more cytochrome b, and less cytochrome c, compared to the M1 population which predominates in cortex. Mitochondria from the red medulla show higher rates of Pi incorporation into total organically bound phosphorus in the presence of ADP, Pi, and oxidizable substrate than do mitochondria from cortex. However, in absence of added oxidizable substrate the reverse is observed, indicating that isolated cortex mitochondria contain more endogenous substrate. The superiority of the red medulla organelles in phosphorylation in the presence of substrate persists in preparations made according to LOWENSTEINS [3] procedure (shortened isolation time, removal of lysosomal enzymes by digitonin treatment). This shows that the observed differences are not artifacts due to different degrees of damage to the organelles by lysosomal attack.