Routes 3 and 4 are minor in the mammary gland relative to the tremendous synthesis and secretion of milk proteins. Uptake of AA's from the blood is adequate to account for the output of AA's in mammary synthesized proteins. For example, intracellular degradation of casein increases as lactation progresses. In addition, proteolysis and remodeling of connective tissue proteins occurs in the basement membrane and stroma, especially during mammary growth and during involution. This involves a complex set of reactions where sugars are put on and taken off of the proteins.
Glycoproteins are very common.
It is very heterogeneously glycosylated. Generally, k -casein from colostrum has a higher carbohydrate content than that from mature milk. It is involved in many regulatory mechanisms. It occurs after completion of the polypeptide chain. It can occur at Ser, Thr, Glu, Asp, His, Lys, and Tyr.