Relationships between radioimmunoassays of alpha lactalbumin and prolactin in bovine skim milk.

A radioimmunoassay developed for alpha-lactalbumin was sensitive between 5 and 140 ng alpha lactalbumin. Addition of increasing volumes of milk to assay tubes progressively decreased binding of iodine-125-labeled alpha-lactalbumin to the antisera in a manner which paralleled the binding curve generated by increasing concentrations of standard alpha-lactalbumin. The addition of 10, 20, 30, or 40 ng of alpha-lactalbumin to diluted milk samples gave 90, 100, 105, and 102% recoveries. Alpha-lactalbumin antisera did not crossreact with 1000 ng of bovine casein, blood serum albumin, beta-lactoglobulin, prolactin, or growth hormone. Milk from each of approximately 100 Holstein Friesian cows at different stages of lactation was sampled monthly for 12 mo. Concentrations of alpha-lactalbumin (1.63 mg/ml) and prolactin (24.9 ng/ml) in samples of skim milk collected in the 1st mo of lactation were greater than those in the remaining months of lactation. Monthly concentrations of alpha-lactalbumin and prolactin in skim milk did not change significantly during seasons of the year. The correlation between concentrations of prolactin and alpha-lactalbumin pooled within subclasses of month of lactation within month of year was .08 for 1125 pairs.