The identification of secreted carbonic anhydrase VI as a constitutive glycoprotein of human and rat milk

In addition to essential nutrients, human milk contains several classes of bioactive factors such as enzymes, hormones, and growth factors, many of wh ich are implicated in infantile growth and development. Secretory carbonic anhydrase isoenzyme VI (CA VI) has been identified earlier as an essential component of mammalian saliva, and we demonstrate here by using biochemical and immunohistochemical techniques that it is also an elementary component of milk. The 42-kDa glycopolypeptide purified from human milk in CA inhibi tor affinity chromatography shared 100% homology with salivary CA VI in the protein sequence analysis (40% coverage), and its digestion with PNGase F resulted in a polypeptide backbone similar in size to salivary CA VI. Quant ification of CA VI in milk by using a time-resolved immunofluorometric assa y revealed an approximately eight-times-higher concentration in human colos trum than in mature milk, the latter corresponding to the levels previously detected in human saliva. The high concentration in the colostrum, in part icular its functional and structural stability in an acidic milieu, and its growth-supporting role in the taste buds suggest that milk CA VI is an ess ential factor in normal growth and development of the infant alimentary tra ct.