Onchocerca retinol- and ivermectin-binding protein activity.

The presence of retinol-binding protein (RBP) activity in Onchocerca cervicalis adult worms and interaction with ivermectin has been studied using high pressure size exclusion chromatography (HPSEC). Four distinct peaks of [3H]-retinol incorporation were obtained corresponding to approximate molecular weights of 150, 67, 19.7 and 4.6 kDa, the 2 smaller M(r) peaks accounting for most of the binding activity. Competition for binding using non-labelled retinol at 200-fold molar excess indicated that specific binding of retinol occurred only to the 19.7 kDa fraction. Competition by ivermectin also inhibited binding of [3H]-retinol to the third peak. Following incubation with [3H]-ivermectin 4 peaks of similar molecular weights were also detected by HPSEC in soluble adult worm homogenate. However, in this case the 150 kDa fraction was most prominent. Both non-labelled ivermectin and non-labelled retinol at 200-fold molar excess reduced binding of [3H]-ivermectin to all 4 fractions. These data indicate that the putative Onchocerca RBP has an approximate molecular weight of 19.7 kDa, that retinol also binds to 3 additional fractions non-specifically, that the pattern of binding of ivermectin to adult worm material is quantitatively and qualitatively different from the binding exhibited by retinol, and that ivermectin interferes with the binding of retinol to the 19.7 kDa Onchocerca protein.