Hormonal regulation of rat mammary gland enzyme activities and metabolite patterns

Activities of nine enzymes and the levels of 15 metabolites were determined in mammary glands from normal (N) and adrenalectomized–ovariectomized (AO) rats in midlactation. Mammary gland weights and mammary DNA did not change between day 11 and day 19 of lactation in N animals and were not affected appreciably by AO. Four days after AO the activities of citrate cleavage enzyme, malic enzyme, UDPglucose pyrophosphorylase, the A protein of the lactose synthetase complex, and glucose-6-phosphate dehydrogenase were decreased to 24, 59, 53, 52, and 56% of day 11 N. These same enzymes require cortisol therapy to increase in activity after adrenalectomy on the 5th day of lactation. Activities of phosphoglucomutase, 6-phosphogluconate dehydrogenase, fructose-1,6-diphosphate aldolase, and isocitrate dehydrogenase were not affected by AO. These enzymatic changes are discussed in relation to systems regulating milk synthesis. Mammary glucose levels increased 1.9-fold in AO animals. Levels of triose phosphate, α-glycerol phosphate, and aspartate in AO animals were decreased to 52, 49, and 62% of N. The proportion of high-energy phosphate bonds to total adenine nucleotides was not affected by AO. Ratios of [free NADP]/[free NADPH2] were not significantly affected by AO or stage of lactation. Metabolite patterns are discussed in relation to changes in enzyme activities and in vitro tracer data.

Intercellular contact and cell-surface galactosyl transferase activity (cell culture-mouse-radioautography-contact inhibition-cis-and trans-galactosylation)

Evidence is presented suggesting the presence of galactosyl transferases and galactosyl acceptors on the outer surfaces of intact Balb/c 3T3 cells. In addition, the data indicate that these transferases may only be capable of transferring galactose from uridine diphosphate galactose to galactosyl acceptors on adjacent cells after intercellular contact is made (trans-glycosylation). Intact Balb/c 3T12 cells, by contrast, show no requirement for intercellular contact in order to carry out this reaction suggesting that these cells, which do not exhibit contact inhibition of growth, may be able to transfer galactose to acceptors situated on the same cell as the enzyme (cis-glycosylation). Electrophoretic and radioautographic assays were used to detect surface transferase activities in these two cell lines. Results of experiments on cells from sparse and dense cultures, and under conditions where intercellular contact was regulated, are consistent with the above hypothesis.

[Glucuronidation of hydroxy-compounds of testosterone and androstenedione by the microsomal UDP glucuronyl transferase of rat liver]

The microsomal UDP glucuronyl transferase exhibits activities against hydroxy derivatives of androstenedione (hydroxyl groups in the positions 2β, 6β or 16α) between 5% and 27% of the extent shown against testosterone. 2β-, 6β· and 16α-hydroxyl groups are much less efficient in accepting the glucuronic acid than the 17β-hydroxyl group.

However, the acceptor function of the 17β-hydroxyl group is restricted by other hydroxy substituents in the testosterone molecule to an increasing extent represented by the following sequence: 2α, 6β, 6α, 16α, and 7α. A special case is represented by 2β-hydroxy-testosterone. The transferase displays a higher activity against this compound than against testosterone.

Apparently the transferase approaches the steroid molecule from the α-side (with the β-side there is also contact at the C-6 atom) requires the 17β-hydroxyl group and the 3-oxo-4-ene system to display full activity.

Thus the very high specificity of the transferase for testosterone explains the selective action of this enzyme on testosterone metabolism in the liver. This action is expressed by the fact, that in liver perfusates the percentage of testosterone in the glucuronide fraction is twice as large as the percentage of testosterone in the free steroid fraction.

Bilirubin excretion in rats with normal and impaired bilirubin conjugation: effect of phenobarbital

The effect of phenobarbital on bilirubin excretion was studied in rats with different capacities for bilirubin conjugation. Drug treatment induced substantial increases in bilirubin UDP-glucuronyl transferase activity in the liver of both normal and heterozygous Gunn rats, but not homozygous Gunn rats in which enzyme activity is completely absent. However, enhancement of bilirubin excretion in vivo was observed only in heterozygous Gunn rats. In these animals the maximum capacity to excrete bilirubin into bile (T(max)), like the activity of the conjugating enzyme, was half normal; phenobarbital caused an increase in T(max) to levels characteristic of normal animals, with a twofold rise in the excretion of conjugated pigment. This appeared to be largely unrelated to enhancement of bile flow, and there was no stimulation of alternate pathways of bilirubin excretion. Conjugated bilirubin was consistently recovered from the plasma and urine of both untreated normal and heterozygous Gunn rats infused with unconjugated pigment. The quantities thus recovered comprised a similar fraction of the total pigment conjugated in both types of animal. Moreover, there were linear correlations between T(max) and both the rate of bile flow and the activity of the conjugating enzyme over the range of values represented by control rats of both types. These findings suggest that the process by which conjugated bilirubin is secreted into the bile is closely related to conjugation and limits the final excretory rate at different levels of pigment excretion. The phenobarbital effect uniquely observed in heterozygous Gunn rats appears to be mediated primarily by enhancement of the limited capacity for bilirubin conjugation with an associated rise in functional secretory capacity.

The heterogeneity of uridine diphosphate glucuronyltransferase from rat liver

1. The glucuronide conjugation of p-nitrophenol, phenolphthalein, o-aminophenol and 4-methylumbelliferone by rat liver microsomes has been studied. The detergent Triton X-100 activated UDP-glucuronyltransferase activity towards all these substrates, therefore the optimum activating concentration was added in all experiments. 2. Mg(2+) enhanced the conjugation of the substrates. 3. With phenolphthalein substrate inhibition occurred but this could be relieved by adding albumin, which binds excess of phenolphthalein. 4. Kinetic constants of the substrates and UDP-glucuronate have been determined. Mutual inhibition was found with the substrates p-nitrophenol, 4-methylumbelliferone and phenolphthalein. p-Nitrophenol conjugation was inhibited competitively by phenolphthalein and 4-methylumbelliferone. 5. o-Aminophenol did not inhibit the conjugation of the other three substrates because these are conjugated preferentially to o-aminophenol. 6. It is concluded that the four substrates are conjugated by one enzyme at the same active site.

Bilirubin glucuronyltransferase. Specific assay and kinetic studies

1. Bilirubin glucuronide was synthesized in vitro in a system containing a rat liver microsomal fraction, UDP-glucuronic acid, Mg(2+) and bilirubin. The enzymic synthesis was accomplished without the addition of a bilirubin carrier. 2. Azobilirubin and azobilirubin glucuronide were separated by t.l.c. and paper chromatography and the measurement of the conjugate provided a specific assay for bilirubin UDP-glucuronyltransferase (EC 2.4.1.17). 3. This diazo compound was labelled when [U-(14)C]UDP-glucuronic acid was employed in the transglucuronidation reaction. 4. Identity of the glucuronide nature of the product was further confirmed by hydrolysis with beta-glucuronidase prepared from limpets and Helix pomatia. In each instance azobilirubin and glucuronic acid were liberated. 5. There was a close correlation between the bilirubin glucuronyl-transferase activity as measured by two procedures, colorimetric and radioisotopic. The specific activities so measured were 19nmol of bilirubin ;equivalents' conjugated/h per mg of protein and 16.9-18.4nmol of UDP-glucuronic acid incorporated/h per mg of protein, respectively. On this basis, it was concluded that the major product formed in vitro was bilirubin monoglucuronide; this represents about 77% of the total products formed. 6. The K(m) values for bilirubin and UDP-glucuronic acid at pH8.2 are 3.3x10(-4)m and 1.67x10(-3)m, respectively. 7. The addition of Mg(2+) at a final concentration of 5mm to the reaction mixture increased the rate of conjugation by 5.6-fold in the microsomal preparation that had been subjected to overnight dialysis against 10mm-EDTA (disodium salt). 8. Diethyl-nitrosamine at a final concentration of 1-20mm has no effect on the glucuronidation of bilirubin in vitro.

The phospholipid-dependence of uridine diphosphate glucuronyltransferase. Reactivation of phospholipase A-inactivated enzyme by phospholipids and detergents

Specific degradation of the phospholipid membrane of guinea-pig liver microsomal fraction with phospholipase A inactivated glucuronyltransferase. The inactivation was reversed by phosphatidylcholine and mixed microsomal phospholipid micelles at concentrations similar to those present in intact microsomal preparations. The other commonly occurring phospholipids did not reactivate phospholipase A-treated enzyme. Since the mixed microsomal phospholipids consisted mainly of phosphatidylcholine, it is concluded that the reactivation by phospholipids is phosphatidylcholine-specific. Reactivation was also achieved by low concentrations of the cationic detergents cetylpyridinium chloride and cetyltrimethylammonium bromide. Higher concentrations of these detergents inactivated the glucuronyltransferase activity of intact and phospholipase A-treated microsomal fractions. Anionic detergents were potent inactivators of the glucuronyltransferase activity of untreated and phospholipase A-treated microsomal fractions, whereas non-ionic detergents had little effect on the activity of either preparation. Measurements of the zeta-potentials of the micellar species used in this study showed that no obvious relationship existed between the zeta-potentials and the ability to reactivate glucuronyltransferase. However, high positive or negative zeta-potentials were correlated with the ability of the amphipathic compound to inactivate glucuronyltransferase.

Mechanism of polymerization of the Salmonella O-antigen: utilization of lipid-linked intermediates

Cell envelope fractions from Salmonella can utilize exogenous lipid-linked intermediates for the synthesis of polymeric O-antigen. We describe a method for preparing aqueous suspensions of lipid intermediates and show that freezing and thawing of cell envelope-lipid intermediate mixtures is required for efficient synthesis. The lipid intermediates move freely in the hydrophobic environment of the membrane.

Intracellular distribution of -glucuronidase activity during metamorphosis of the housefly, Musca domestica L

β-Glucuronidase and glucuronyl transferase activities were determined in various stages of third instar larvae and pupae of M. domestica. Most of the β-glucuronidase activity was found in the particulate fractions: lysosomes and microsomes. No enzyme activity could be detected in the early third instar larvae. It increased by 10-fold in the pupal stage and has a pH optimum of 4.0. The enzyme activity in the soluble fraction increased from 30% in the late larval stage to 45% in the late pupal stage. About 56% of lysosomal β-glucuronidase activity exhibited structure-linked latency. The free activity of the lysosomal fractions increased from 44% in the late larval stage to over 64%, in the late pupal stage. Glucuronyl transferase activity was low at all times during metamorphosis. The rapid rate of β-glucuronidase synthesis during pupal and late pupal stages and the increase in the lysosomal free activity has been discussed in relation to the cellular dissolution and tissue reconstitution during metamorphosis.