Complete sequence analysis of rat transferrin and expression of transferrin but not lactoferrin in the digestive glands.

Rat milk and digestive juices contain transferrin but not lactoferrin, which is a major iron-binding protein in these secretions of human and mouse. To compare the structure of rat transferrin to that of transferrins and lactoferrins in other species, we isolated a cDNA clone containing the entire coding region of transferrin from rat liver and determined its sequence. The amino-acid sequence of rat transferrin had 69.8% identity with that of human transferrin and 48.8% identity with that of human lactoferrin. Rat transferrin, like other transferrins, had the potential N-linked glycosylation site only in the C-terminal domain, although lactoferrins characterized so far contained the glycosylation sites in both the N- and C-terminal domains. Southern and Northern analyses showed that there was the gene specifically hybridized with the mouse lactoferrin cDNA in rat genomic DNA, but only the transferrin mRNA was detected in mammary gland, submaxillary gland and pancreas of rat. These results suggest that the rat lactoferrin gene is silent in the mammary gland, and transferrin can serve as a functional substitute for lactoferrin in rat.