Insulin-like growth factor-I and its association with binding proteins in bovine milk.

Immunoreactive insulin-like growth factor-1 (IGF-I in bovine milk was quantified. IGF-I was principally assoication with an approximately 45 kDa binding protein. In addition, a small fraction of IGF-I occurred at a molecular weight approximately the same as that of unbound IGF-I. Available binding sites existed on the approximately 45 kDa binding protein. Bound IGF-I was readily dissociated from binding protein by acid treatment. When IGF-I was estimated in milk obtained from primiparous and multiparous cows, mutiparous cows had a higher concentration (40 nmol/1) [corrected] at parturition than primiparous cows (19.2 nmol/1) [corrected]. By day 2 of lactation, IGF-I concentrations were 30 and 50% of initial estimates for multiparous and primiparous cows respectively. the final IGF-I concentration, on day 56 of lactation, was 4.5 nmol/1 [corrected] for combined parity groups. At parturition in multiparous cows, the mass of IGF-I was estimated at 183 and 157 nmol [corrected] for blood and milk pools respectively. Milk, therefore, represents a substantial pool of IGF-I in the cow. The mechanism of the appearance of IGF-I in bovine milk is unknown.