THE BEHAVIOR OF HUMAN VITRONECTIN IN-VIVO - EFFECTS OF COMPLEMENT ACTIVATION, CONFORMATION AND PHOSPHORYLATION

We examined the behaviour in vitro of native, specifically phosphoryla ted, and multimeric vitronectin to determine the effects of these modi fications on its turnover. distribution and molecular behaviour. In no rmal rabbits, the plasma half-life (T-1/2) of antigenically detected v itronectin was 8.00+/-1.26 h (mean+/-s.d.), with a fractional cataboli c rate (FCR) of 18.77+/-1.57%/h and extravascular/intravascular ratio (EV/IV) of 1.00 (0.45-1.60, median and range). For vitronectin selecti vely phosphorylated by protein kinase A, T-1/2 was 8.87+/-0.48 h, with a significantly smaller FCR of 10.85+/-0.71%/h (P < 0.005) and an EV/ IV of 0.28 (0.15-0.36) (P < 0.05 compared with antigenically detected vitronectin). In vitro, phosphorylation had no effect on the affinity of vitronectin for heparin-Sepharose, while complement activation with cobra venom factor (CVF) led to a two-ford enrichment of P-32-vitrone ctin within the SC5b-9 complex. In vivo CVF caused a rapid decrease in the circulating levels of P-32-vitronectin and was accompanied by the prompt appearance of a high mol. wt species consistent with SC5b-9. D espite specific incorporation of P-32-vitronectin into SC5b-9, both fo rms of the molecule had similar inhibitory effects on C9-mediated haem olysis of EAC1-7 cells. Urea-activated vitronectin was rapidly cleared from circulation with less than 15% remaining after 1 h while protein -bound label accumulated in the spleen, lung and liver. These results demonstrate that vitronectin is a rapidly metabolized protein whose in vivo behaviour is markedly altered when phosphorylated or activated t o form multimers and SC5b-9.