VITREOUS AND AQUEOUS HUMORS CONTAIN A LATENT PROTEINASE ACTIVITY THATABOLISHES IGF BINDING TO SPECIFIC IGF BINDING-PROTEINS

Several distinct insulin-like growth factor binding proteins (IGFBPs) are present in tissues and fluids of the developing and adult eye. How ever, the mechanism(s) involved in the regulation of ocular IGFBP leve ls is unknown. We have now identified an endogenous factor in vitreous and aqueous humors that, when activated by sodium dodecyl sulfate (SD S), abolishes the capacity of specific low molecular weight IGFBPs (i. e. 24-30 kDa) to bind IGF as assessed by western ligand blotting. In c ontrast, IGF binding to the 46 and 32 kDa IGFBPs (IGFBP-3 and IGFBP-2 respectively) is not affected by the SDS-activated inhibitory factor ( IF). Maximal activation of the IF occurs at an SDS concentration of ap proximately 0.015%. Incubations in the presence of the serine-proteina se inhibitor aprotinin result in marked inhibition of IF activity. Pre liminary characterization by ultrafiltration suggests that the IF is l arge (<100 kDa) and/or that it is present in a complex. The finding of a factor, most likely a serine proteinase, that specifically abolishe s IGF binding to low molecular weight IGFBPs suggests a mechanism for regulating the levels of these IGFBPs and thus the functional activiti es of IGFs in ocular fluids under normal and/or pathological condition s.