Allosteric activation of rat liver microsomal [hydroxymethylglutaryl-CoA reductase (NADPH)]kinase by nucleoside phosphates

Microsomal 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase kinase activity is enhanced about 5 fold by 2 mM of either AMP or ADP. Activation constants, Ka, for AMP and ADP are 17 microM and 430 microM respectively, showing that AMP is a more potent activator than ADP. This property is expressed by increasing not only the rate of reductase inactivation but also the rate of reductase phosphorylation from [gamma-32P]ATP. GTP can replace ATP as substrate of reductase kinase but GMP and GDP cannot replace AMP as activators. Kinetic studies show that ATP can only act as a substrate. Nucleoside mono or diphosphates and nucleoside triphosphates, thus, appear to bind to different sites on microsomal HMG-CoA reductase kinase. Nucleoside mono or diphosphates act as allosteric activators of reductase kinase. The adenosyl moiety and the unaltered phosphate ester at the 5' position are two essential features of the activator molecule. Phosphorylation of reductase either by microsomal or cytosolic AMP-activated reductase kinase produces an 80% inactivation, with a concomitant incorporation of 0.8 mol of 32P per mol of reductase (Mr 55,000). In both cases exhaustive tryptic digestion of 32P-labeled HMG-CoA reductase, which had been denatured in 2M urea, yields two major phosphopeptides, the phosphoryl group being bound to serine residues.     

Affinity labeling of the catalytic and AMP allosteric sites of 3-hydroxy-3-methylglutaryl-coenzyme A reductase kinase by 5'-p-fluorosulfonylbenzoyladenosine

The nucleotide analogue 5'-p-fluorosulfonylbenzoyladenosine (FSBA) reacts irreversibly with rat liver cytosolic 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase kinase, causing a rapid loss of the AMP activation capacity and a slower inactivation of the catalytic activity. The rate constant for loss of AMP activation is about 10 times higher (kappa 1 = 0.112 min-1) than the rate constant of inactivation (kappa 2 = 0.0106 min-1). There is a good correspondence between the time-dependent inactivation of reductase kinase and the time-dependent incorporation of 5'-p-sulfonylbenzoyl[14C]adenosine ([14C]SBA). An average of 1.65 mol of reagent/mol of enzyme subunit is bound when reductase kinase is completely inactivated. The time-dependent incorporation is consistent with the postulate that covalent reaction of 1 mol of SBA/mol of subunit causes complete loss of AMP activation, whereas reaction of another mole of SBA/mol of subunit would lead to total inactivation. Protection against inactivation by the reagent is provided by the addition of Mg2+, AMP, Mg-ATP, or Mg-AMP to the incubation mixtures. In contrast, addition of ATP, 2'-AMP, or 3'-AMP has no effect on the rate constants. Mg-ATP protects preferentially the catalytic site against inactivation, whereas Mg-AMP at low concentration protects preferentially the allosteric site. Mg-ADP affords less protection than Mg-AMP to the allosteric site when both nucleotides are present at a concentration of 50 microM with 7.5 mM Mg2+. Experiments done with [14C]FSBA in the presence of some protectants have shown that a close correlation exists between the pattern of protection observed and the binding of [14C]SBA. The postulate is that there exists a catalytic site and an allosteric site in the reductase kinase subunit and that Mg-AMP is the main allosteric activator of the enzyme.     

5-aza-C-induced changes in the time of replication of the X chromosomes of Microtus agrestis are followed by non-random reversion to a late pattern of replication

Treatment with 5-azacytidine (5-aza-C) causes an advance in the time of replication and enhances the DNase-I sensitivity of the inactive X chromosome in Gerbillus gerbillus fibroblasts. We found that these changes were not stably inherited and upon removal of the drug the cells reverted to the original state of one active and one inactive X chromosome. In order to determine whether this reversion was random, we used a cell line of female Microtus agrestis fibroblasts in which the two X chromosomes are morphologically distinguishable. In this work we show that the reversion to a late pattern of replication is not random, and the originally late replicating X chromosome is preferentially reinactivated, suggesting an imprinting-like marking of one or both X chromosomes. The changes in the replication pattern of the X chromosome were associated with changes in total DNA methylation. Double treatment of cells with 5-aza-C did not alter this pattern of euchromatin activation and reinactivation. A dramatic advance in the time of replication of the entire X linked constitutive heterochromatin (XCH) region was however, observed in the doubly treated cells. This change in the replication timing of the XCH occurred in both X chromosomes and was independent of the changes observed in the euchromatic region. These observations suggest the existence of at least two independent regulatory sites which control the timing of replication of two large chromosomal regions.Deceased on 2 Jan. 1987