Relationships between Radioimmunoassays of Alpha Lactalbumin and Prolactin in Bovine Skim Milk

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Abstract
A radioimmunoassay developed for .alpha.-lactalbumin was sensitive between 5-140 ng .alpha.-lactalbumin. Addition of increasing volumes of milk to assay tubes progressively decreased binding of 125I-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 month 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.