Ccxxiv. the Metabolism of Galactose. Ii. the Synthesis of Lactose by Slices of Active Mammary Gland in Vitro

ONE of the most interesting natural transformations in carbohydrate metabolism is that occurring in the active mammary gland during the synthesis of the galactose-containing disaccharide, lactose. Rohmann [1919] was able to obtain some evidence of the synthesis of lactose by the action of an extract of the mammary gland upon added sucrose. This investigator, on what now seems inconclusive evidence, suggested a stereokinase system, present in the blood as well as the gland and capable of causing the interconversions: glucose -* fructose -+ galactose. More recently, Michlin and Lewitow [1934], using the minced mammary gland of lactating cows, have obtained an enzymic synthesis from added glucose and galactose of a disaccharide, which is hydrolysed by emulsin and is presumably lactose. Bert [1884] and later Porchet [1909] claimed that in animals which had previously had the mammary gland removed and subsequently became pregnant, hyperglycaemia developed and the sugar appearing in the urine was not lactose but glucose. Widmark and Carlens [1925] reported hypoglycaemia in lactating cows during milking, the degree roughly corresponding to the amount of milk produced. Paton and Cathcart [1911] obtained evidence to indicate that the glucose of the blood is used in the mammary gland during the formation of lactose. Harding and Downs [1929] were unable to demonstrate a lowered blood sugar in freely lactating women while on a hospital diet. They expressed the view that the amount of glucose circulating in the blood is controlled by the internal secretions rather than conditioned entirely by the lactose output of the active mammary gland. Nitzescu [1925] has been able to show that in fasting there is a drop in the lactose concentration of the milk produced by animals in full lactation. Upon the intravenous injection of glucose, galactose, fructose or maltose, the normal level of the milk lactose is restored. Injection of sucrose or lactose was without effect-these sugars are not utilised since there is no effective concentration of sucrase or lactase in the blood-and the disaccharides administered are, for the most part, excreted into the urine, owing to the low renal threshold for these sugars. The results of Nitzescu are explicable equally well by the assumption of a direct synthesis of lactose from the sugars administered by the active mammary gland, or by the previous conversion of these sugars into glucose, by way of glycogen formation in the liver. Thus the available physiological evidence suggests that the freely lactating gland is apparently capable of withdrawing glucose from the blood and converting it into the milk sugar, lactose. The gland is automatically supplied with a surplus of blood sugar to meet the demands of milk formation, the stimulus emanating from some other source than the mammary glands themselves.