Technical note: a radioimmunoassay for porcine retinol binding protein

This report describes a RIA for porcine retinol binding protein (RBP) that has been validated for measurement of RBP concentrations as low as 10 ng/mL in serum, follicular fluid, uterine flushings, allantoic fluid, and endometrial and uterine tissue culture medium. The increasing displacement of porcine [125I]RBP resulting from decreasing dilutions of each sample was parallel to the standard curve, and exogenous RBP added to samples was accurately measured with the assay. To examine specificity, 1-mL samples of serum, uterine flushings, follicular fluid, and allantoic fluid were subjected to G-100 Sephadex chromatography, and the fractions were assayed for RBP with the RIA and for fluorescence, which detects retinol bound to RBP. The RBP measured with the RIA and the fluorescent fractions coeluted. Samples of uterine flushings in which RBP was measured using immunoblotting and densitometry were also assayed with the RIA. The measurements using the two methods were correlated (r = .84, P < .01). These results are evidence that the assay is specific for RBP. The limit of detection of the assay was .46 ng. The RIA should facilitate the study of RBP function and the control of its secretion by various tissues in pigs.