The potential role of butyrophilin and xanthine oxidoreductase in controlling the amount and size of milk-fat droplets

Abstract

The secretion of fat droplets from mammary epithelial cells requires the expression of two major proteins, butyrophilin1A1 (BTN) and xanthine oxidoreductase (XOR). Ablation of the BTN or XOR gene in mice results in the accumulation of large fat droplets suggesting a reciprocal relationship between BTN/XOR concentration and milk-fat droplet size. We tested this hypothesis by correlating BTN/XOR concentrations in cow and mouse with their fat droplet size. The amount of BTN in mouse was 75 times less than in bovine samples. The size of fat droplets in mice was larger than in cow, but no correlation was found between fat-droplet size and the amount of BTN/XOR. Experimental reduction in fat-droplet size in mice did not change the concentration of BTN. We propose that a low amount of BTN is sufficient to mediate its role in milk-fat secretion and that it may have additional functions to its potential role as a structural protein.