A Theory of Protein Metabolism in Man

Abstract

During a 24-hour period when the subject is maintained in nitrogen balance with either protein, or a mixture of amino acids, or a single amino acid, the larger fraction (60-75%) of the nitrogen metabolized in this period is derived not from the nitrogen ingested during this interval, but from that already present. In this sense the bulk of the protein metabolized in any one day is endogenous. The term endogenous is used here in a different sense from the connotation given it by Folin, who used it to designate the wear and tear quota of protein metabolized. The simplest direct evidence for the above conclusion was furnished by experiments in which a mixture of amino acids and protein was ingested containing only a small amount of sulphur. These experiments showed that under these conditions there is no sparing of the basal sulphur. In several instances more sulphur was excreted than the sum of the amount ingested and the total excreted during a fasting period of 24 hours. It follows therefore that when protein is ingested the nitrogen and sulphur excreted do not come for the most part from the ingested protein; but that approximately three quarters is derived from protein (or protein split products) already in the organism.