Affinity labeling of the folate-methotrexate transporter from Leishmania donovani

Confirmation of the role of N-acetyltransferase 2 in teratogen-induced cleft palate using transgenics and knockouts

Previous work on Dilantin- and hydrocortisone-induced cleft palate and cleft lip with or without cleft palate using congenics for the N-acetyltransferase loci (Nat1 and Nat2 are closely linked) and recombinant inbred lines implicated the Nat1,2 region in susceptibility to teratogen-induced orofacial clefting. Since Nat1 does not differ between the two strains, Nat2 appeared to be responsible. We have now tested this conclusion using transgenics and knockouts. Transgenics for human NAT1 (equivalent to mouse Nat2) and knockouts for Nat2 were tested for susceptibility to Dilantin, hydrocortisone, and 6-aminonicotinamide-induced orofacial clefting. We found that Nat2 greatly influences teratogen-induced orofacial clefting on the A/J background but not on the C57BL/6J background. The magnitude and direction of the effects depended on which teratogen was used. The Nat2 knockout did not make C57BL/6J susceptible or A/J (already with very low activity) more susceptible but significantly decreased sporadic clefting in the A/J strain. We conclude that only the A/J strain, with several loci affecting orofacial clefting, is influenced by Nat2.

New perspectives on folate catabolism

Folate catabolism has been assumed to result from the nonenzymatic oxidative degradation of labile folate cofactors. Increased rates of folate catabolism and simultaneous folate deficiency occur in several physiological states, including pregnancy, cancer, and when anticonvulsant drugs are used. These studies have introduced the possibility that folate catabolism may be a regulated cellular process that influences intracellular folate concentrations. Recent studies have demonstrated that the iron storage protein ferritin can catabolize folate in vitro and in vivo, and increased heavy-chain ferritin synthesis decreases intracellular folate concentrations independent of exogenous folate levels in cell culture models. Ferritin levels are elevated in most physiological states associated with increased folate catabolism. Therefore, folate catabolism is emerging as an important component in the regulation of intracellular folate concentrations and whole-body folate status.

Epidermal N-acetylation of p-aminobenzoyl glutamic acid: difference in response to ultraviolet B irradiation

Arylamine N-acetyltransferases (EC 2.3.1.5) are conjugating drug-metabolizing enzymes and consist of two major isozymes, NAT1 and NAT2. As they have different substrate specificity and expression of polymorphism, distribution of the isozymes may be detected by investigating acetylation. p-Aminobenzoyl glutamic acid (pABG), one of the specific substrates for NAT1 in human pro-monocytic cell-line, was metabolized through acetylation by 9000 x g supernatant fraction from human epidermal keratinocytes and the effect of ultraviolet B (UVB) irradiation on the acetylation was also studied. Forty-eight hours after irradiation of UVB (200 J/m2), the activity was not increased (114 +/- 8.3%, n = 3), while increase in N-acetylating capacity for 2-aminofluorene (substrate for NAT1 and NAT2) amounted to 201 +/- 16% (n = 3). These results suggest that there are at least two isozymes for N-acetylation in the human epidermic and NAT2 may be affected by UVB irradiation.

Required allosteric effector site for N-acetylglutamate on carbamoyl-phosphate synthetase I

Carbamoyl-phosphate synthetase I (CPSase I) catalyzes the entry and rate-limiting step in the urea cycle, the pathway by which mammals detoxify ammonia. One facet of CPSase I regulation is a requirement for N-acetylglutamate (AGA), which induces an active enzyme conformation and does not participate directly in the chemical reaction. We have utilized labeling with carbodiimide-activated [14C]AGA to identify peptides 120-127, 234-237, 625-630, and 1351-1356 as potentially being near the binding site for AGA. Identification of peptide 1351-1356 confirms the previous demonstration (Rodriquez-Aparicio, L. B., Guadalajara, A. M., and Rubio, V.(1989) Biochemistry 28, 3070-3074) that the C-terminal region is involved in binding AGA. Identification of peptides 120-127 and 234-237 constitutes the first evidence that the N-terminal region of the synthetase is involved in ligand binding. Since peptides 631-638 and 1327-1348 have been identified near the ATP site of CPSase I (Potter, M. D., and Powers-Lee, S. G.(1992) J. Biol. Chem. 267, 2023-2031), the present finding of involvement of peptides 625-630 and 1351-1356 at an "allosteric" activator site was unexpected. The idea that portions of the AGA effector site might be derived from an ancestral glutamine substrate site via a gene duplication and diversification event was considered.

Acetylation of p-aminobenzoylglutamate, a folic acid catabolite, by recombinant human arylamine N-acetyltransferase and U937 cells

N-acetyl-p-aminobenzoylglutamate is a major urinary metabolite of folic acid. It is formed by acetylation of p-aminobenzoylglutamate following cleavage of the C9-N10 bond of folic acid. Using recombinant human type 1 (NAT1) and type 2 (NAT2) arylamine N-acetyltransferase, we have shown that p-aminobenzoylglutamate is a specific NAT1 substrate. At an acetyl-CoA concentration of 50 microM, the Km for p-aminobenzoylglutamate (pABG) acetylation by recombinant NAT1 was 130 +/- 13 microM. For the human pro-monocytic cell-line U937, the apparent Km was slightly higher (333 +/- 17 microM). Inhibitor studies supported NAT1-dependent acetylation of pABG by U937 cell cytosols. These studies are the first to identify a potential endogenous substrate for human NAT1 and suggest that this enzyme may be important in the cellular clearance of pABG.

Biopterin conversion to reduced folates by Leishmania donovani promastigotes

The ability of Leishmania donovani promastigotes to proliferate in folate-deficient medium supplemented with pterins suggests that pterins can serve as a source of folate in these parasites [16]. Using reversed-phase high-performance liquid chromatography, the ability of intact L. donovani to transform [3H]biopterin into tetrahydrofolates was demonstrated. Radioactivity was primarily associated with 5-methyltetrahydrofolate and 10-formyltetrahydrofolate. A mutant strain of L. donovani, MTXA5, that was genetically deficient in folate transport capacity and incapable of growing in pterin-supplemented folate-deficient growth medium, exhibited a greatly reduced capacity to metabolize [3H]biopterin to reduced folates. These data indicated that wild-type L. donovani promastigotes, unlike mammalian cells, were able to convert biopterin to tetrahydrofolates and supported the hypothesis that folate transport deficiency in mutant organisms is associated with an inability to transform pterins to reduced folates.

Nutritional requirements of wild-type and folate transport-deficient Leishmania donovani for pterins and folates

The nutritional requirements for folates and pterins were assessed for two strains of Leishmania donovani promastigotes, a wild-type (D1700) parental strain and a mutant derivative (MTXA5) which possesses a markedly diminished capacity to transport both [3H]folate and [3H]methotrexate (MTX). Both L. donovani strains have an absolute growth requirement for an exogenous pterin, since their proliferation could not be sustained in completely defined medium lacking either a pterin or a folate. Supplementation of the growth medium by many of a spectrum of folates and pterins could support growth of the wild-type cell line. Surprisingly, however, the MTXA5 strain could not thrive in folate-deficient medium fortified with any of the pterins that promoted wild-type cell division, including biopterin and neopterin. The relationship between the incapacity of MTXA5 cells to transport folate and methotrexate and their inability to multiply in folate-deficient medium supplemented with pterins was evaluated using genetic approaches. First, an independently generated MTX-resistant mutant, MTXB4, was isolated in 1 mM MTX. The phenotype of the MTXB4 cells was like that of MTXA5 cells since they were unable to transport [3H]MTX and [3H]folate or grow in folate-deficient medium supplemented with pterins. Second, three revertants of the MTXA5 cells were selected for their ability to grow in 1 microM biopterin. All three revertants had regained [3H]folate and [3H]MTX transport capability concomitant with growth sensitivity to methotrexate toxicity. These observations provide genetic evidence that the competence of L. donovani to transport [3H]folate and [3H]MTX and the capability of the organisms to utilize pterins as a nutritional factor are influenced by a common genetic locus.