Compartmentalization of fucosyltransferase and alpha-L-fucosidase in human milk

Several pathogenic agents of pediatric gastroenteritis are inhibited by fucosylated oligosaccharides of human milk. Biosynthesis and degradation of fucosyloligosaccharides is controlled by fucosyltransferase and alpha-L-fucosidase. The activity of these enzymes varies reciprocally over the course of lactation. We hypothesized that differences in the specific organization of these enzymes in compartments of human milk might contribute to such differences in activity. Therefore, the distribution of these enzymes in various compartments of human milk was investigated. After ultracentrifugation at 120,000g for 2h, the fucosyltransferase activity distributes evenly between the supernatant and the membranous pellet. Ultracentrifugation at 180,000g for 17 h further fractionated the milk into a clear supernatant, a fluff layer from the supernatant, and a pellet. The fluff was visualized by electron microscopy. The distribution of fucosyltransferase activity in colostrum was compared with that of mature milk from the same donor. In mature milk from Day 30 of lactation, most fucosyltransferase activity was in the membranous fluff fraction, while in colostrum from Day 5 of lactation, most of the fucosyltransferase activity was in the supernatant. In contrast to fucosyltransferase, fucosidase activity was found only in the soluble milk fraction; upon prolonged ultracentrifugation, most of this was membrane associated. The nature of human milk fucosidase was studied. This enzyme is glycosylated and exhibits several characteristics common to other fucosidases. Under the conditions found in human milk, it exhibits almost full activity. The variation in compartmentalization of fucosyltransferase activity during lactation may reflect variations in metabolism of the fucosyloligosaccharides of human milk that protect against disease.