Hepatic Protein Metabolism in the Infant Rat

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Abstract
Although growth is the most striking change in the development from neonate to adult, there are more subtle changes during this interval, for example, in enzyme levels. This investigation was undertaken to study changes in protein metabolism of the rat during preweaning development. The rate and extent of incorporation of 14C-leucine into protein of the whole liver and of the liver microsomes increased to age 11 days, the neonate having the lowest rate of incorporation. The total pool of free leucine also increased with age. When compared per unit of DNA, RNA or wet weight, there were two peaks in the free leucine pool. There was a small peak at 7 days of age and a much larger peak at 19 days of age. Cathepsin activity was found to decrease in all cell fractions with age, the greatest decrease occurring within 24 hours of birth. The rate of protein turnover was found to be lower in the younger rats; this may be interpreted to indicate either that protein metabolism in the infant rat is directed almost entirely toward anabolism, or that leucine is reutilized at a higher rate.