Rats malnourished since birth and fed on a protein-free diet for 2 weeks showed a 23-27% decrease in the State-3 oxidation of glutamate, succinate and ascorbate + NNN′ N′-tetramethyl-p-phenylenediamine by liver mitochondria compared with control fed animals. ATP synthesis and the respiratory control index were diminished at the three coupling sites, but significant alterations were not observed in ADP/O ratios. Vmax. for NADH oxidation in electron-transport particles was 40% lower. Mitochondrial cytochromes b and c1 remained unchanged, but cytochrome c was increased by 26%. Cytochromes a + a3 were diminished by 22%. Vmax. for mitochondrial ATPase was 23% lower. These results suggest that the lower content of cytochrome a + a3 at the rate-controlling step of oxidative phosphorylation in malnourished rats might be mainly responsible for the decrease in substrate oxidations as well as ATP synthesis at the three coupling sites. The decreased synthesis and hydrolysis of ATP suggests that other energy-dependent mitochondrial processes could be decreased during malnutrition.

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