An ultraviolet spectrophotometric assay for aspartate transcarbamylase

The why behind the high: determinants of neurocognition during acute cannabis exposure

Acute cannabis intoxication may induce neurocognitive impairment and is a possible cause of human error, injury and psychological distress. One of the major concerns raised about increasing cannabis legalization and the therapeutic use of cannabis is that it will increase cannabis-related harm. However, the impairing effect of cannabis during intoxication varies among individuals and may not occur in all users. There is evidence that the neurocognitive response to acute cannabis exposure is driven by changes in the activity of the mesocorticolimbic and salience networks, can be exacerbated or mitigated by biological and pharmacological factors, varies with product formulations and frequency of use and can differ between recreational and therapeutic use. It is argued that these determinants of the cannabis-induced neurocognitive state should be taken into account when defining and evaluating levels of cannabis impairment in the legal arena, when prescribing cannabis in therapeutic settings and when informing society about the safe and responsible use of cannabis.

A continuous spectrophotometric assay for aspartate transcarbamylase and ATPases

A new continuous coupled uv-spectrophotometric assay is described for two phosphate-releasing enzymes, aspartate transcarbamylase and ATPase of herpes simplex virus (HSV). Phosphate release is coupled to the phosphorolysis of the nucleoside analog 7-methylinosine (m7Ino) catalyzed by purine nucleoside phosphorylase. When this reaction is monitored at 291 nm, the coupled assay can readily detect 10 nmol Pi released/min. Our method offers advantages over a recently reported continuous assay devised for measuring aspartate transcarbamylase activity using the nucleoside analog methylthioguanosine (MESG) as the linking substrate. In contrast to MESG, m7Ino is easily and inexpensively synthesized and is also commercially available. The spectrophotometric signal at 291 nm, produced by the difference in the extinction coefficients between nucleoside substrate and the base product, is significant over a much wider pH range than the signal difference between MESG and its phosphorolysis product at 360 nm. Saturation curves for aspartate and carbamyl phosphate and pH rate profiles have been reproduced using the purine nucleoside phosphorylase/m7Ino coupled assay. Initial velocity patterns constructed over micromolar to millimolar concentrations of aspartate and carbamyl phosphate yielded four kinetic parameters simultaneously. To further illustrate the application of this coupled assay, kinetic parameters were determined for the DNA-dependent ATPase reaction of HSV helicase-primase.

Boundary spreading in sedimentation velocity experiments on partially liganded aspartate transcarbamoylase. A ligand-mediated isomerization

Transport theory for rapidly reversible interacting systems was used to analyze boundary spreading in sedimentation velocity experiments on partially liganded aspartate transcarbamoylase. In the presence of sub-stoichiometric amounts of a bisubstrate analog, N-(phosphonacetyl)-L-aspartate, which is bound with high affinity to the enzyme (Kd approximately 100 nM), broad boundaries were observed consistent with the presence of two conformational forms. The theoretical treatment showed that under these conditions, the interconversion between the compact (11.7 S) and swollen (11.3 S) forms of the enzyme appears uncoupled, due to the formation of a gradient of free ligand that is caused by the re-equilibration resulting from the differential sedimentation of the two enzyme forms. Sedimentation velocity patterns for such systems are interpretable in terms of two independent species. When, however, the enzyme is in the presence of a sub-saturating amount of the weakly bound ligand, succinate (Kd approximately 1 mM), the re-equilibration caused by the differential sedimentation does not perturb the large background of free ligand and form a gradient. Instead, the two different forms of the enzyme are in dynamic equilibrium, resulting in a boundary having average sedimentation and diffusion coefficients. The observed boundary spreading experiments with different ligands are satisfactorily interpreted in terms of a ligand-mediated isomerization of aspartate transcarbamoylase from a compact to a swollen conformation.

Shared active sites in oligomeric enzymes: model studies with defective mutants of aspartate transcarbamoylase produced by site-directed mutagenesis

Many oligomeric enzymes are functional only in the assembled form, and it is often difficult to determine unambiguously why monomers are inactive. In some cases individual monomers cannot fold into stable correct ("native") conformations without contributions from interchain interactions. For other oligomers, catalysis requires the contributions of amino acid residues at the interface between adjacent polypeptide chains, and monomers are inactive because they cannot form complete active sites. A test for the presence of shared sites was devised that is based on the formation of active hybrid oligomers from appropriate inactive parental mutants produced by site-directed mutagenesis. This approach was applied in a study of the catalytic trimer of aspartate transcarbamoylase (aspartate carbamoyltransferase, EC 2.1.3.2) from Escherichia coli, using three mutants, in which Ser-52 was replaced by His, Lys-84 by Gln, or His-134 by Ala. Hybrid trimers formed from the virtually inactive Ser and Lys mutants were 10(5) more active than the parental proteins, and the specific activities of each hybrid were about 33% that of the wild-type trimer, as expected for the scheme based on shared sites. Hybrids from the His and Lys mutants had comparable specific activities. Moreover, one hybrid with approximately 33% activity had one high-affinity binding site for a bisubstrate analog as compared to about three for wild-type trimer. As a further test, hybrids were also formed from wild-type and double-mutant (Lys-84----Gln and His-134----Ala) trimers. The hybrid containing two chains with the double mutation and one wild-type chain had very little activity, and that composed of one double mutant and two wild-type chains had 32% the specific activity of wild-type trimers. This negative complementation experiment is in quantitative accord with the scheme based on shared sites at or near the interfaces between adjacent chains. The techniques used to demonstrate shared active sites in the catalytic subunits of aspartate transcarbamoylase can be applied generally to various types of oligomers (dimers, tetramers, etc.) to determine whether the participation of amino acid residues from adjoining chains is essential for forming active sites in oligomeric enzymes.

Regeneration of active enzyme by formation of hybrids from inactive derivatives: implications for active sites shared between polypeptide chains of aspartate transcarbamoylase

Crystallographic studies of Escherichia coli aspartate transcarbamoylase (aspartate carbamoyltransferase, EC 2.1.3.2) in conjunction with chemical modification experiments have led to the suggestion that the active sites of the enzyme are at the interfaces between adjacent polypeptide chains of the catalytic trimers and involve joint participation of amino acid residues from the adjoining chains. However, the precise locations of the active sites and of the residues involved in catalysis are not known. To test the hypothesis that the active sites are shared between chains, we constructed hybrid trimers in which two chains were modified at one presumed active site residue and the third chain was altered at a different active site residue. One parental trimer was a reduced pyridoxal phosphate derivative in which lysine-84 was modified and the other was a mutant protein in which tyrosine-165 was converted to serine by site-directed mutagenesis. Incubating mixtures of these two virtually inactive derivatives under conditions promoting interchain exchange led to a large increase in enzyme activity corresponding approximately to the formation of one active site per trimer. The purified hybrid trimers, containing either two pyridoxylated and one mutant chain or vice versa, had 23% and 28%, respectively, the activity of native wild-type catalytic trimers, compared to 5% and 3% for the parental trimers. The most likely explanation for this large increase in activity is the formation of one "native" active site in each of the hybrid trimers. The results constitute strong evidence for shared active sites in aspartate transcarbamoylase.

On conformational changes in the regulatory enzyme aspartate transcarbamoylase

Our preliminary studies provide clear answers to only some of the basic questions posed above regarding the nature of conformational changes in ATCase. By exploiting the reverse reaction to develop a kinetic active site titration technique we demonstrated that symmetry-related regions of ATCase are affected equally in the allosteric transition even though the bisubstrate analog is bound to only one of the six active sites. This is a vivid illustration of a "global" change in an enzyme, and it provides powerful support for the view that active site ligands promote a concerted transition of the enzyme from the low-activity to the high-activity conformation. Kinetic experiments show that this conformational change is rapid, requiring only tenths of a second. In contrast, the studies with isolated catalytic subunits provide evidence for a much more local conformational change which occurs after the ligand is bound and which is much slower, lasting over a time period of many seconds. Although the experiments on the holoenzyme do not show directly how many atoms are involved in the conformational change or how large are the displacements, they do indicate, especially in conjunction with other studies, that many amino acid residues in both the catalytic and regulatory chains must be implicated in the T----R allosteric transition. Why PALA has such a different local effect on the catalytic subunit than does the combination of carbamoylphosphate and succinate is not clear, but this uncertainty may be resolved by further NMR studies. Determining how the local changes at the active sites are linked to the global transition affecting the quaternary structure of the enzyme remains a formidable problem. Also further work is needed in order to determine whether unliganded ATCase molecules exist in an equilibrium mixture of T and R conformations even prior to the ligand binding event.