RADIOIMMUNOASSAY FOR INSULIN-LIKE GROWTH FACTOR-I IN BOVINE-MILK

This experiment was designed (1) to study the effectiveness of acid-ethanol extraction and gly-gly HC1 incubation to dissociate the IGF-binding protein complex before IGF-I RIA measurements in bovine milk, and (2) to determine the influence of freezing/thawing and defatting of milk samples on IGF-I concentration of milk. Acidification of milk with acid-ethanol (acid-ETOH) liberated more IGF-I from its binding proteins than with gly-gly HC1. Milk samples obtained from three control cows and three rbST-treated cows (41-44 wk postpartum) were treated with acid-ethanol and assayed by a radio-immunoassay. IGF-I concentrations in the milk were 3.30 +/- 1.40 for the control cows and 2.77 +/- 1.36 ng mL-1 for the rbST-treated cows. Fat removal before or after freezing did not affect milk IGF-I measurements. The milk concentration of IGF-I after freezing/thawing twice was not different from that after freezing/thawing once. The radioimmunoassay resulting from this study should provide a tool for monitoring IGF-I concentrations in milk throughout the entire lactation.