Alanosine and hadacidin--comparison of effects on adenylosuccinate synthetase

Cavitation as a mechanism of substrate discrimination by adenylosuccinate synthetases

Adenylosuccinate synthetase catalyzes the first committed step in the de novo biosynthesis of AMP, coupling L-aspartate and IMP to form adenylosuccinate. Km values of IMP and 2'-deoxy-IMP are nearly identical with each substrate supporting comparable maximal velocities. Nonetheless, the Km value for L-aspartate and the Ki value for hadacidin (a competitive inhibitor with respect to L-aspartate) are 29-57-fold lower in the presence of IMP than in the presence of 2'-deoxy-IMP. Crystal structures of the synthetase ligated with hadacidin, GDP, and either 6-phosphoryl-IMP or 2'-deoxy-6-phosphoryl-IMP are identical except for the presence of a cavity normally occupied by the 2'-hydroxyl group of IMP. In the presence of 6-phosphoryl-IMP and GDP (hadacidin absent), the L-aspartate pocket can retain its fully ligated conformation, forming hydrogen bonds between the 2'-hydroxyl group of IMP and sequence-invariant residues. In the presence of 2'-deoxy-6-phosphoryl-IMP and GDP, however, the L-aspartate pocket is poorly ordered. The absence of the 2'-hydroxyl group of the deoxyribonucleotide may destabilize binding of the ligand to the L-aspartate pocket by disrupting hydrogen bonds that maintain a favorable protein conformation and by the introduction of a cavity into the fully ligated active site. At an approximate energy cost of 2.2 kcal/mol, the unfavorable thermodynamics of cavity formation may be the major factor in destabilizing ligands at the L-aspartate pocket.

Determinants of L-aspartate and IMP recognition in Escherichia coli adenylosuccinate synthetase

Adenylosuccinate synthetase governs the first committed step in the de novo synthesis of AMP. Mutations of conserved residues in the synthetase from Escherichia coli reveal significant roles for Val(273) and Thr(300) in the recognition of l-aspartate, even though these residues do not or cannot hydrogen bond with the substrate. The mutation of Thr(300) to alanine increases the K(m) for l-aspartate by 30-fold. In contrast, its mutation to valine causes no more than a 4-fold increase in the K(m) for l-aspartate, while increasing k(cat) by 3-fold. Mutations of Val(273) to alanine, threonine, or asparagine increase the K(m) for l-aspartate from 15- to 40-fold, and concomitantly decrease the K(i) for dicarboxylate analogues of l-aspartate by up to 40-fold. The above perturbations are comparable with those resulting from the elimination of a hydrogen bond between the enzyme and substrate: alanine mutations of Thr(128) and Thr(129) increase the K(m) for IMP by up to 30-fold and the alanine mutation of Thr(301) abolishes catalysis supported by l-aspartate, but has no effect on catalysis supported by hydroxylamine. Structure-based mechanisms, by which the above residues influence substrate recognition, are presented.

Reduction of adenine nucleotide content of clone 4 MDCK cells: effects on multiplication, protein synthesis, and morphology

The antitumor agent hadacidin (N-formyl-hydroxyamino-acetic acid), at 4 mM, inhibited the multiplication of clone 4 Madin Darby canine kidney (MDCK) cells within 24 hr. Growth resumed rapidly upon replacement of hadacidin with aspartate, an observation consistent with the drug's action as a competitive inhibitor of adenylosuccinate synthetase, an enzyme in adenine nucleotide biosynthesis. Data indicate that the drug-treated cells were arrested in S phase of the cell cycle. Accompanying inhibition of multiplication was a 16-fold increase in the area occupied by the cells and a refractoriness to release by treatment with trypsin. None of these changes occurred when 0.5 mM adenosine was included in the incubation mixture containing the inhibitor. Hadacidin decreased the adenosine triphosphate (ATP) and cyclic adenosine monophosphate (cAMP) content of the cells as well as the rate at which 3H-leucine was incorporated into protein. In the presence of 1 mM dibutyryl cAMP and theophylline, the drug had no effect on cell division and protein synthesis. The data suggest that, in clone 4 MDCK cells, the effects of hadacidin are mediated by diminishing the level of cAMP.

A Phase I study of the combination N-(phosphonacetyl)-L-aspartate (PALA, NSC-224131) and L-alanosine (NSC-153353) in patients with advanced cancer

This article reports a Phase I study of combined therapy with N-(phosphonacetyl)-L-aspartate (PALA) and L-alanosine in 26 patients with advanced cancer. Each agent exhibits antitumor effect by enzyme inhibition: PALA blocks pyrimidine biosynthesis by impeding aspartate transcarbamylase and L-alanosine depletes purine nucleotides by interfering with adenylosuccinate synthetase. These agents were selected for clinical investigation in light of synergistic cytotoxicity in vitro against human tumor cell lines and in vivo against P-388 murine leukemia resistant to cytosine arabinoside. Dose-limiting toxicities were stomatitis and diarrhea to a lesser extent. There was no substantial myelosuppression. The authors recommend either of two intravenous regimens for studies of therapeutic activity in selected patients with neoplastic diseases: a one-day treatment repeated of PALA, 5.0 g/m2 and L-alanosine, 3.0 g/m2, repeated every 3 weeks; or a monthly program of PALA, 500 mg/m2/d 1-5 and L-alanosine, 60 mg/m2/d 1-5.

Phase I study of L-alanosine (NSC 15353)

L-alanosine (NSC 15353) is a newly developed antitumor antibiotic which acts as an inhibitor of purine intermediary metabolism. Experimental antitumor activity was demonstrated in a variety of murine neoplasms. A Phase I trial was undertaken on a daily x 5 (d x 5) schedule in 22 evaluable patients. Dose limiting toxicity was an oral mucositis characterized by beefy red oral, lingual and pharyngeal erythema. The maximum tolerated dose is 320 mg/m2/d x 5 every three weeks. The recommended dose for Phase II evaluation is 160 mg/m2/d x 5 every three weeks.

The potentiation of adenine toxicity to Chinese hamster cells by coformycin: suppression in mutants with altered regulation of purine biosynthesis or increased adenylate-deaminase activity

When added to medium containing coformycin (2 microM or above), adenine is lethal to Chinese hamster fibroblasts at the concentration inhibiting de novo purine biosynthesis (Debatisse and Buttin, '77b). Rescue by hypoxanthine suggested that cells die of IMP starvation when the analog can turn off deamination of both adenosine and adenylate. As predicted from this hypothesis, two classes of variants resistant to the mixture of coformycin + adenine have been isolated: Class 1 variants have altered control of de novo IMP biosynthesis; they fall into two subclasses on the basis of their resistance to adenosine. Class 2 variants have a 6-10-fold increased level of AMP-deaminase (E.C.: 3.5.4.6); their growth in the selective medium is temperature-dependent, a property accounted for by the observation that cell growth in the presence of coformycin imposes a gradual thermodependent decay of specific AMP-deaminase activity in both wild-type and variant lines. This control by coformycin of AMP-deaminase activity is unaltered in mutants deficient in the four activities of adenosine-kinase. APRT, HGPRT and deoxycytidine-kinase. Most of the resistant variants are unstable and exhibit either increased or reduced resistance, depending on prolonged growth in selective or normal medium.

Determination of L-alanosine in plasma and urine by reversed-phase high-performance liquid chromatography of the Dns derivative

L-Alanosine is an antitumour antibiotic that has recently been placed in clinical trial. We have developed a relatively rapid and specific assay for urinary and plasma alanosine, based on formation of the Dns derivative and separation of this from other Dns compounds by reversed-phase high-performance liquid chromatography. Dns-Alanosine is detected by its absorption at 254 nm, since alanosine is atypical in that it forms a Dns derivative with very low fluorescence. The lower limit of detection of alanosine in plasma is 0.1 microgram/ml. The assay has been used to measure the levels of alanosine in the plasma and urine of rabbits and of man.

Adenosine kinase as a new selective marker in somatic cell genetics: isolation of adenosine kinase--deficient mouse cell lines and human--mouse hybrid cell lines containing adenosine kinase

A new selective system for isolating somatic cell hybrids, using adenosine kinase as the selective marker, has been developed. The selective medium for forward selection (to select for cells containing adenosine kinase) contains alanosine, adenosine and uridine. To survive in the presence of alanosine, cells must have adenosine kinase in order to utilize exogenous adenosine as the sole source of AMP. Uridine is added to the selective medium to prevent the toxic effects of adenosine on cultured mammalian cells. The selective medium for reverse selection (to select for cells lacking adenosine kinase) contains 2-fluoroadenosine, an analogue of adenosine, which is converted to a toxic nucleotide by the action of adenosine kinase. Mouse mutant cell lines deficient in adenosine kinase have been derived. Human--mouse hybrid cells containing the kinase have been prepared from one of these mutant lines. Karyotype data of these hygrid lines and their adenosine kinase-minus sublines are consistent with assignment by others of the human gene for adenosine kinase on chromosome 10.

Regulation of purine metabolism. Adenylosuccinate synthetase from Novikoff ascites tumor cells

Adenylosuccinate synthetase has been partially purified from Novikoff ascites tumor cells. The properties of the protein are quite different from the enzyme from rat liver in that the Km for asparate is higher and the K1 for the feedback inhibitor AMP is also higher. The antibiotic hadacidin has a preferential inhibitory effect on the tumor enzyme. These results suggest that the Novikoff ascites tumor enzyme is less sensitive to normal feedback controls but may be more sensitive to specific antitumor drugs.