Studies of the Metabolism of Asialotransferrins: the in Vivo Behavior of Baboon and Rhesus Asialotransferrins

The catabolism and distribution of rhesus and baboon asialotransferrins relative to the corresponding parent proteins were studied in rabbits using a dual isotope tracer technique. Also a similar study with the baboon proteins in a baboon is reported. The metabolic data obtained in rabbits with both rhesus and baboon transferrins was close to the values established in a previous study for rabbit transferrin. Desialylation resulted in an average increase in the fractional catabolic rate of rhesus trasferrin by 22.7%. This change is similar to that found earlier with asialotransferrins from several nonprimate mammals which are thought not to interact with the hepatic asialoglyco-protein receptor. Two kinetically distinct fractions were identified in baboon asialotransferrin. One of these, amounting to approximately one-third of the protein, was eliminated from the circulation very rapidly. The remaining two-thirds constituted a slowly catabolized fraction which behaved in vivo similarly to rhesus asialotransferrin. Unlike the rapidly cleared fraction, elimination of the slowly catabolized fraction in baboon asialotransferrin is probably not mediated by the hepatic asialoglycoprotein receptor. An amount comparable to the rapidly eliminated fraction in baboon asialotransferrin was recovered with the liver of rats in short-term experiments. In rats which were preinjected with chicken acid alpha1-glycoprotein the hepatic uptake of baboon asialotransferrin was markedly reduced. Data obtained in the baboon agreed with the findings in rabbits, although transferrin turnover was slower in the baboon. From its behavior in vivo as an asialoglycoprotein, baboon transferrin shows greater resemblance to human transferrin than rhesus transferrin. The conclusion is supported by carbohydrate analyses which show an intermediate position for baboon transferrin between man and a nonprimate mammal (rabbit), and a similarity between rhesus and rabbit transferrins.