Insulin in Bovine Colostrum and Milk: Evolution Throughout Lactation and Binding to Caseins

¹ Tecnología y Bioquímica de los Alimentos, Facultad de Veterinaria, Miguel Servet, 177, 50013 Zaragoza, Spain
² Technología y Bioquímica de los Alimentos, Facultad de Veterinaria, Miguel Servet, 177, 50013 Zaragoza, Spain

The changes in insulin concentration in bovine milk in the first period of lactation and its association with other milk proteins were studied. Highest concentration was found in the first milking (327 ng/ml). This concentration fell within the first 24 h postpartum to about 50% of its initial value. By d 3, the level was about 25%, and, on d 7, a stable concentration was reached at approximately 46 ng/ml (about 14% of its initial value). This concentration is about 100 times higher than that in serum, which suggests a specific mechanism of transfer from blood to milk. Colostral whey obtained by ultrafiltration or ultracentrifugation contains much less insulin than colostrum. When colostrum or milk was incubated with [¹²⁵I]insulin and whey and casein fractions were separated by precipitation, it was observed that most insulin remained with the casein. However, when colostrum was incubated with [¹²⁵I]insulin and subjected to gel filtration, most of the radioactivity corresponded to free insulin, indicating that insulin is associated with the precipitated casein but not with the casein micelles in solution.

Key Words: insulin • colostrum • casein

Submitted on March 19, 1991
Accepted on July 8, 1991