Quantitative aspects of glycine metabolism in the rabbit

Abstract

Tracer doses of [alpha]-C14 glycine were given to rabbits and the radioactivity of the free glycine in plasma, liver and muscle measured at intervals from 5 minutes and 18 hours after injection. In some experiments free serine was isolated from liver and muscle and assayed for radioactivity. Glutathione, the mixed proteins of liver and muscle and the mixed serum proteins were hydrolysed and the radioactivity of the resulting glycine measured. The plasma glycine decreased sharply in radioactivity during the first 30 minutes, less in the next few hours and very slowly from 6 to 18 hours after injection. Apparently dilution of the labeled plasma glycine resulted from several reactions of widely differing rates. Exchange of plasma glycine with liver glycine was fast, the calculated time required for complete replacement being between 15 and 20 minutes. Permeability barriers apparently played only an insignificant part in this exchange, the limiting factor being rate of blood-flow. The exchange between plasma glycine and muscle glycine was very much lower, the "renewal time" with this tissue being 20-25 hours. This slow equilibration was caused both by the low rate of blood-flow in muscle and by the existence of a permeability barrier. Comparison of the radioactivities of free glycine and free serine indicated a very fast and reversible conversion of glycine into serine. The appreciably higher specific radioactivity of muscle serine than of muscle glycine suggested that the muscle cell was more permeable to serine than to glycine. The "renewal times" of glutathione were about 10-12 hours in the liver and 20 hours in muscle. The bearing of the present results on calculations of protein turnover are considered and the various assumptions discussed. Incorporation of labeled amino acid into tissue protein depends on rates of blood-flow, cell penetration and protein synthesis. Calculations based on certain assumptions indicated that in the rabbit approximately 1.9 g of protein were synthesized in the liver/day/kg, about 40% as plasma proteins and 60% as liver proteins. Although low incorporation of labeled glycine into muscle proteins was caused in part by the low rate of penetration of glycine into muscle, the rate of protein synthesis in muscle was concluded to be much smaller than in liver. However, no quantitative deductions can be made.