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Turnover Rates of Various Muscle Proteins^{1, 2,}

University of Wisconsin — Madison^,4, Madison 53706

Abstract

Nitrogen equilibrium may be established at almost any desired level of nitrogen intake from as low as 150 mg per kilogram per day to 800 mg per kilogram per day or more. It was thought at one time that only a small fraction of this nitrogen was utilized by the body for replacement of nitrogen lost in the wear and tear of body tissues. The bulk of protein was simply used as an energy source and the nitrogen excreted as urea. According to Folin's classical distinction between exogenous and endogenous nitrogen metabolism, the bulk of excreted nitrogen was of an exogenous origin and did not arise from the metabolism of tissue protein (Folin, 1905).

Such a distinction between endogenous and exogenous metabolism was challenged by the pioneer work of Schoenheimer and his colleagues which showed an extensive incorporation of isotopically labeled dietary components into all organs and tissues (Schoenheimer, 1942). One must conclude, therefore, that the nitrogenous constituents of the diet promptly enter into the nitrogen metabolism of the body tissues and become indistinguishable from similar substances already present.

¹ This work was supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison, Wisconsin and by a grant from NIH, AM-14704. It is taken in part from the Ph.D. Thesis of Harriet Song, University of Wisconsin-Madison, August, 1973. We are grateful to Mr. Terry Burk for invaluable technical assistance.

² Invitational paper presented at the Symposium on Protein Synthesis and Muscle Growth held during the 65th Annual Meeting of the American Society of Animal Science, Lincoln, Nebraska, July 28 to August 1, 1973.

³ Present address: C/O College of Agriculture, Seoul National University, Suwon, Korea.

⁴ Department of Nutritional Sciences.