The behaviour of human vitronectin in vivo: effects of complement activation, conformation and phosphorylation

We examined the behaviour in vivo of native, specifically phosphorylated, and multimeric vitronectin to determine the effects of these modifications on its turnover, distribution and molecular behaviour. In normal rabbits, the plasma half-life (T1/2) of antigenically detected vitronectin was 8.00 +/- 1.26 h (mean +/- s.d.), with a fractional catabolic rate (FCR) of 18.77 +/- 1.57%/h and extravascular/intravascular ratio (EV/IV) of 1.00 (0.48-1.60, median and range). For vitronectin selectively phosphorylated by protein kinase A, T1/2 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-fold enrichment of 32P-vitronectin within the SC5b-9 complex. In vivo CVF caused a rapid decrease in the circulating levels of 32P-vitronectin and was accompanied by the prompt appearance of a high mol. wt species consistent with SC5b-9. Despite specific incorporation of 32P-vitronectin into SC5b-9, both forms of the molecule had similar inhibitory effects on C9-mediated haemolysis 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 to form multimers and SC5b-9.