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Immunoreactive Prolactin in Subcellular Fractions from Bovine Mammary Tissue¹

Department of Animal Sciences, Purdue University, West Lafayette, IN 47907

ABSTRACT

Mammary tissue from lactaring Holsteins at slaughter was homogenized and fractionated into subcellular compartments. Bovine prolactin was quantified in each subcellular fraction and in pre-slaughter milk by radioimmunoassay procedures validated for this purpose. Prolactin concentrations were ng prolactin/mg protein for all samples. Immuno-reactive prolactin was in rough endoplasmic reticulum (24.8 ng/mg), Golgi apparatus (18.8 ng/mg), and secretory vesicle (10.2 ng/mg). All of these concentrations exceeded that in samples of milk (2.7 ng/mg). Nuclear fractions contained a material that reacted in the prolactin radioimmunoassay, but criteria for valid quantification could not be satisfied. These results confirm prolactin inside bovine mammary cells, and they are consistent with an intracellular pathway for transfer of prolactin from plasma into milk.

¹ Journal Paper No. 9118, Purdue University Agricultural Experiment Station. Supported in part by grant GM 23889 from National Institutes of Health.

² Department of Biochemistry and Nutrition, Virginia Tech, Blacksburg, VA 24061.