Metabolism of rabbit angiostatin glyco-forms I and II in rabbits: Angiostatin-I leaves the intravascular space faster and appears to have greater anti-angiogenic activity than angiostatin-II

Plasminogen (PLG) exists in the circulation as two glycoforms, I and II. An giostatin (AST) is a polypeptide that has been cleaved from the kringle reg ion of PLG and has strong anti-angiogenic properties. AST-I and AST-II, whi ch consisted only of kringles I through 3, were prepared by the action of u rokinase on purified rabbit PLG-l and PLG-II, respectively, in the presence of N-acetyl cysteine, followed by affinity chromatography on lysine-Sephar ose. Purified AST-I and AST-II were tested for functional activity with a c hick chorioallantoic membrane (CAM) model; when similar amounts were applie d to a 6-day CAM, AST-I was substantially more effective than AST-II in dec reasing vascular supply to the CAM over a 72-hour period; this activity cor related with a loss of capillaries, probably through apoptosis of endotheli al cells. Radiolabeled AST-I and AST-II (iodine 125 and iodine 131) were co -injected intravenously into healthy rabbits to determine their clearances from plasma measured over 3 days. Over a dose range of 0.08 to 2.7 mug/kg, the fractional catabolic rate within the intravascular space (j(3)) indicat ed that AST-I was cleared 3-fold to 4-fold more rapidly than AST-II (P < .0 01). The catabolic half-life of AST-I (2.01 +/- 0.19 days) was significantl y less than that of AST-II (2.62 +/- 0.20 days). The faster clearance of AS T-I from the intravascular space was matched by its more rapid passage than AST-II to the extravascular space of various organs over 60 minutes in viv o. This property of AST-I as compared with AST-II may partially explain its greater anti-angiogenic potential. From the plasma concentrations of PLG-I and PLG-II and their relative behaviors toward rabbit VX-2 lung tumors in vivo, we predict that substantially greater quantities of AST-II than AST-I may be released into the extravascular space of tumors.