Kinetic Analysis of Synthesis and Secretion of Plasma Proteins in a Marine Teleost

Abstract

Abstract The time dependence of appearance of radioactive protein in plasma of the toadfish, Opsanus tau, following rapid injection of 14C-labeled amino acids via the hepatic portal vein, has been studied at two temperatures in fish subjected to acute temperature change or to thermal acclimation. At 20° labeled protein appears in plasma after a lag of about 1 hour and accumulates exponentially with a half-time of 1.0 to 1.2 hours. Corresponding time constants at 10° are 6 hours and approximately 8 hours, however accumulation follows an S-shaped curve at this temperature. At the termination of the experiments plasma protein accounts for about 20% of total incorporation into protein by liver. Labeling patterns following intraperitoneal injection are slower and more complicated in shape. Turnover half-lives for 14C-labeled plasma proteins are 9 days for fish at 20° and about 16 days at 10°. Acclimation to cold (10°), a process of metabolic adjustment which causes about 50% elevation in protein synthetic rate in liver, produces only small changes in the time course and level of incorporation of amino acids into circulating plasma proteins. A kinetic model for the synthetic-secretory pathway in liver is proposed, involving three temperature-dependent processes: a migration phase which causes an absolute delay in secretion of labeled protein; a stage of collection or saturation of an intermediate compartment which produces a period of slow rise in the labeling curve; a rapid accumulation and plateau phase described by a set of reversible reactions for synthesis and discharge. Rate constants for the model are derived from the present data and previous results, which yield theoretical curves consistent with the observed kinetics, temperature dependency, and thermal acclimation effect.