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Xanthine oxidoreductase (XOR), a key enzyme of purine metabolism, has been implicated in the secretion of the milk fat droplet in lactating mammary epithelial cells, possibly through structural interactions with other milk fat globule proteins including butyrophilin (Btn) and adipophilin (ADPH). To help determine the mechanism by which XOR is regulated, we examined the expression and localization of XOR in the non-secretory states of late pregnancy and induced involution compared with the state of active secretion. XOR mRNA levels started to increase at mid-pregnancy, turned sharply upwards at the onset of lactation and decreased rapidly with forced involution, indicating transcriptional control of the enzyme level by differentiation and secretory function. During pregnancy and involution the enzyme was diffusely distributed in the cytoplasm, but moved rapidly to the apical membrane of the cells when secretion was activated, where it colocalized with both Btn and ADPH, similar to the situation in the milk fat globule itself. Size-exclusion chromatography of solubilized milk fat globule membrane proteins showed that XOR formed a sulphydryl-bond-dependent complex with Btn and ADPH in the milk fat globule membrane. XOR returned to a diffuse cytoplasmic localization shortly after induced involution, while Btn remained localized to the apical membrane, suggesting that localization of XOR is not dependent on the presence of Btn in the apical membrane. Our findings indicate that the expression and membrane association of XOR in the mammary gland are tightly regulated by secretory activity, and suggest that the apical membrane association of XOR regulates the coupling of lipid droplets to the apical plasma membrane during milk lipid secretion.
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