Respiratory Enzymes in the Liver of the Newborn Rat

The respiratory enzymes of the cell are known to be located in the mitochondria and are of fundamental importance in vital processes. During the first few days of life great changes have been found in the activity of these enzymes in the liver of the rat. At birth the rates of oxidation by homogenates for all substrates were found to be low. There was a three- to fourfold increase in activity in the first few days of life, at which time the adult level of activity was reached. Tissue analysis and histochemical studies have revealed the underlying mechanism for this process. At birth individual liver cells were found to have a low popula­tion of mitochondria, which increased rapidly in step with the respiratory activity. Detailed study of the individual enzymes of the respiratory chain has revealed a similar pattern of development, with the exception of the enzyme DPNH-cytochrome c reductase. This enzyme lags considerably behind in its development ; adult activity is not reached until several weeks after birth. Diphosphopyridine nucleotide and cytochrome c were also present in low concentrations at birth and increased considerably during the first week of life. Mitochondria isolated from foetal liver have been found to possess different properties from those of adult liver. They show considerable stimulation of respiration on the addition of diphosphopyridine nucleotide (DPN) ; a phenomenon known as the 'DPN effect’ and also seen in mitochondria isolated from tumours. They also fail to maintain their oxidative ability when stored at 0 °C in 0·25/M-sucrose. Respiration can be restored by the addition of DPA. Both these phenomena indicate that the mitochondrial membrane is abnormally permeable to DPN. It is proposed that the increases in activity of respiratory enzymes may be related to the assumption of full functional activity by the liver.