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Lactose biosynthesis and relevant enzymatic activity in rabbit mamma ry tissue during various stages of pregnancy and lactation are investigated by using a tissue-slice incubation method in order to understand the temporal relationships. Ovulation was induced in 27 New Zealand white rabbits and they were bred by artificial insemination. Sacrifice occurred on days 15, 24, and 29 of pregnancy, and day 2, 5, 8, 15, and 22 post partum. Nucleic acids were extracted and concentratons of DNA determined spectrophotometrically at 600 nm with diphenylamine reagent and RNA determined with orcinal reagent. The tissue incubations were made with (U-14C) glucose. (14C) lactose was then separated by paper chromatography from unchanged radioactive glucose. Enzyme analysis including determining the activities of phosphoglucomutase, UDP-glucose pyrophosphorylase, and UDP-glucose 4-epimerase. Lactose synthase was determined, as well as, hexokinase. A biphasic adaptation in the rate of lactose synthesis and in the RNA concentration was noted during lactogenesis. The 1st increase in the rate of lactose biosynthes is occurred between days 15 and 24 of pregnancy. A 2nd substantial increase was noted immediately post partum. The overall rate of lactose biosynthesis increased 12-fold from day 24 of pregnancy to day 15 of lactation post partum, and then decreased from 15 to 22 days post partum. The RNA concentration/g wet weight of tissue and the ratio of RNA/DNA closely represented the biphasic ability of the mammary-tissue slice to synthesize lactose. Increases in the activities of UDP-glucose 4-epimerase and lactose synthase were most closely correlated with increases in the rate of lactose biosynthesis. UDP-glucose pyrophosphor ylase activity was unrelated with the ability to synthesize lactose, and hexokinase and phosphoglucomutase activities were variable during pregnancy and lactation. Lactose synthase activity was present by day 15 of pregnancy, but the ability to synthesize lactose was undetected until day 24 of pregnancy.

Metabolic adaptations during lactogenesis. Lactose synthesis in rabbit mammary tissue during pregnancy and lactation