Glutaminase activity of glutamine-dependent carbamoyl-phosphate synthase from rat ascites hepatoma. Regulation by adenosine triphosphate-magensium and magnesium ion

Gene regulatory network analysis identifies MYL1, MDH2, GLS, and TRIM28 as the principal proteins in the response of mesenchymal stem cells to Mg2+ ions

Magnesium (Mg)-based implants have emerged as a promising alternative for orthopedic applications, owing to their bioactive properties and biodegradability. As the implants degrade, Mg2+ ions are released, influencing all surrounding cell types, especially mesenchymal stem cells (MSCs). MSCs are vital for bone tissue regeneration, therefore, it is essential to understand their molecular response to Mg2+ ions in order to maximize the potential of Mg-based biomaterials. In this study, we conducted a gene regulatory network (GRN) analysis to examine the molecular responses of MSCs to Mg2+ ions. We used time-series proteomics data collected at 11 time points across a 21-day period for the GRN construction. We studied the impact of Mg2+ ions on the resulting networks and identified the key proteins and protein interactions affected by the application of Mg2+ ions. Our analysis highlights MYL1, MDH2, GLS, and TRIM28 as the primary targets of Mg2+ ions in the response of MSCs during 1-21 days phase. Our results also identify MDH2-MYL1, MDH2-RPS26, TRIM28-AK1, TRIM28-SOD2, and GLS-AK1 as the critical protein relationships affected by Mg2+ ions. By offering a comprehensive understanding of the regulatory role of Mg2+ ions on MSCs, our study contributes valuable insights into the molecular response of MSCs to Mg-based materials, thereby facilitating the development of innovative therapeutic strategies for orthopedic applications.

Regulation of the mammalian carbamoyl-phosphate synthetase II by effectors and phosphorylation. Altered affinity for ATP and magnesium ions measured using the ammonia-dependent part reaction

We have measured the 'core' mammalian carbamoyl-phosphate synthetase II (CPSII) activity, using NH4Cl as the nitrogen-donating substrate and trapping carbamoyl phosphate as urea through its reaction with ammonium ions. When ATP and magnesium ion concentrations are close to those found in the cell, the substrate saturation curves for ammonia and bicarbonate are hyperbolic, giving Km (NH3) values of 166 microM at high ATP concentrations and 26 microM at low ATP concentrations, while the Km (bicarbonate) is 1.4 mM at both ATP concentrations used. These values for the Km (NH3) are lower than previously reported for CPS II, and closer to the values for the mitochondrial counterpart. The Km for ammonia and bicarbonate are not altered by phosphorylation of the multienzyme polypeptide CAD, which contains the first three enzyme activities of pyrimidine biosynthesis. The CPS II activity is lower with an excess of either ATP or magnesium ions, causing the apparently sigmoid dependence of activity upon ATP concentration to be enhanced at low concentrations of free magnesium ions. The feedback inhibitor, UTP, acts by stabilising a state with a low affinity for magnesium ions and for ATP. In the presence of the activator, 5-phosphoribosyl diphosphate (PRibPP), the enzyme has a higher affinity for magnesium ions and thus the ATP dependence of the activity is hyperbolic. Phosphorylation of CAD similarly activates the CPS II enzyme by increasing the affinity for magnesium ions and by pushing the equilibrium away from the low-affinity UTP-stabilised state. Using our improved assay procedure, we observe a very large activation by PRibPP of carbamoylphosphate synthesis at low concentrations of magnesium ions, and we find that unlike UTP, the activator PRibPP is able to act on the phosphorylated enzyme.

Inactivation of Crithidia fasciculata carbamoyl phosphate synthetase II by the antitumor drug acivicin

In Crithidia fasciculata, carbamoyl phosphate synthetase II, which catalyses the first step of de novo pyrimidine biosynthesis, was separated from aspartate carbamoyltransferase by ammonium sulfate fractionation. The antitumor drug acivicin competitively inhibited the synthetase II activity with respect to L-glutamine, yielding an apparent Ki of 2 microM. In the absence of L-glutamine, acivicin resulted in a selective, time-dependent inactivation of L-glutamine-dependent activity of the enzyme, with an inactivation constant (Kinact) of 100 microM and a minimum inactivation half-time (T) of 0.2 min. L-Glutamine protected the enzyme from inactivation. These results are consistent with a postulate that acivicin is an active site-directed affinity analogue of L-glutamine, achieving irreversible inactivation. The inactivated enzyme retained ammonia-dependent activity. Acivicin stimulated the ammonia-dependent activity by increasing the Vmax value of the enzyme; apparent Km values for ammonia and MgATP were not affected. Differential action of acivicin on the Crithidia and mammalian synthetase II is discussed.

Kinetic properties of carbamoyl-phosphate synthetase II (glutamine-hydrolyzing) in the parasitic protozoan Crithidia fasciculata and separation of the enzyme from aspartate carbamoyltransferase

A high specific activity of carbamoyl-phosphate synthetase II (glutamine-hydrolyzing; EC 6.3.5.5) was demonstrated in extract of the cultured Crithidia fasciculata. The enzyme was separated from aspartate carbamoyltransferase by ammonium sulfate fractionation. Apparent Km for the synthetase for L-glutamine, NH4+, MgATP or bicarbonate was 0.27, 26, 1.7 or 1.7 mM at 2.0% dimethyl sulfoxide plus 0.3% glycerol. 8.6% dimethyl sulfoxide plus 1.4% glycerol decreased Km for L-glutamine to 0.10 mM, while Km for MgATP was unaffected. The higher solvent concentrations made Vmax markedly reduced, yielding the inhibition of the activity. These properties are unique to the Crithidia synthetase, compared with the mammalian enzyme.

In vivo inactivation by acivicin of carbamoyl-phosphate synthetase II in rat hepatoma

The antitumor drug acivicin, L-(alphaS,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid, in vivo irreversibly inactivated carbamoyl-phosphate synthetase II(glutamine-dependent)(EC 6.3.5.5), the first and rate-limiting enzyme of de novo pyrimidine nucleotide biosynthesis, in transplantable rat hepatoma and host liver. With two injections of 0.5 mg acivicin per 100 g body weight to one group and two injections of 5 mg to another group, enzyme activity decreased to 20 and 1% in hepatoma and to 99 and 31% in liver respectively. Aspartate carbamoyltransferase (EC 2.1.3.2) activity was not affected. Acivicin in vitro selectively inactivated glutamine-dependent activity of the synthetase II from the hepatoma and liver, with an inactivation constant (Kinact) of 90 microM and a minimum inactivation half-time (T) of 0.7 min. The inactivation velocity with 10 microM acivicin was 5.0-fold stimulated by 2 mM MgATP and 18.4-fold by 2 mM MgATP plus 16.7 mM bicarbonate. MgATP at 0.5 mM caused half-maximum stimulation of the inactivation velocity. Under in vitro conditions, L-glutamine (1 mM) protected the enzyme from inactivation by 10 microM acivicin. The synthetase activity was protected in vitro by 6 mM concentrations for glycine (84%), L-glutamate (59%) and L-aspartate (51%) and by 0.5 mM UTP (35%) from inactivation by 20 microM acivicin. The results are compatible with the suggestion that acivicin is an active site-directed affinity analog of L-glutamine.