Synthetic Polypeptide Metabolism

Abstract

Summary The metabolic fate in rabbits of intravenously administered, iodine-labeled poly d-Glu55d-Lys39-d-Tyr6 differs markedly from that of its enantiomorph, poly Glu58Lys36Tyr6 (No. 2). Although both polymers are rapidly cleared from the circulation, 131I is slowly excreted in the urine after d-polymer injection, but quite rapidly excreted after l-polymer injection. Only 32% of the d-polymer is degraded by the 19th day, whereas 83% of the l-polymer is degraded by the 7th day. This delay in the degradation of the d-polymer is reflected in the retention of large amounts of the injected material in the liver and in the kidneys. The amount of d-polymer in the liver slowly declines from a maximum of 16% at 5 days, but the amount in the kidney continues to increase for 2 to 3 weeks and ultimately reaches 32% of the injected polymer. Radioautographic studies show that both polymers enter the proximal tubular cells of the kidney: the d-polymer persists, whereas the l-polymer is quickly eliminated. Studies with ureteral ligation to stop glomerular filtration demonstrate that the polymers are filtered through the glomerulus and then reabsorbed. Anephric animals show a slower serum elimination rate for the d-polymer than normal animals. Since the hepatic uptake is unchanged, it is likely that the kidney rapidly extracts a certain population of the d-polymer molecules which are not as readily taken up by the liver. Since ultracentrifugation of the labeled d-polymer prior to injection does not affect its distribution, the basis for this biologic heterogeneity is not aggregation. Chromatographic analyses of the dialyzable radioactivity in the urine and in the homogenates of the liver and kidney clearly show the presence of labeled peptides derived from the degradation of the d-polymer or of the l-polymer. A second external label, 59Fe-ferrocene, confirms the serum elimination and urinary excretion patterns obtained with iodine, and, in general, also confirms the organ retention pattern of the d-polymer. However, significant incorporation of the iron from the ferrocene into hemoglobin and other compounds occurs, and this fact limits the usefulness of the ferrocene label, especially for studies of tissue localization. The prolonged retention of the d-polymer in the organs and its gradual release over a long period of time are postulated to cause immunologic paralysis. This hypothesis explains the apparent lack of immunogenicity of d-polymers under the usual conditions of immunization and the lack of a significant booster effect and the ultimate disappearance of circulating antibody following repeated injections of small amounts of the d-polymer.