Inhibition of indolethylamine-N-methyltransferase by analogs of S-adenosylhomocysteine

S-Adenosylhomocysteine Enhances Hydrogen Peroxide-Induced DNA Damage by Inhibition of DNA Repair in Two Cell Lines

It has been proposed that hyperhomocysteinemia may exert its pathogenic effects largely through metabolic accumulation of S-adenosylhomocysteine (SAH), a strong noncompetitive inhibitor of most methyltransferases. Here, we investigated the effects of SAH on H(2)O(2)-induced cellular DNA damage in comparison with the effects of homocysteine (Hcy) in a mouse endothelial cell line and a human intestinal cell line. Cells were preincubated for 2 h with H(2)O(2) (20 microM) followed by incubation with SAH or Hcy for 3 h. DNA strand breakage was determined using comet assay and DNA repair capacity determined using the same assay over time at 1, 2, and 3 h during SAH incubation. In both types of cells, SAH at 0.25-2 microM strongly and dose dependently enhanced H(2)O(2)-dependent DNA damage and inhibited DNA repair, whereas Hcy had a much weaker effect. SAH markedly increased uracil misincorporation, and this effect was also much stronger than that of Hcy. Taken together, our results show that SAH potentiates H(2)O(2)-induced DNA damage in cell cultures through impaired DNA repair capability and suggest that such effects are related to uracil misincorporation. Although the in vivo relevance of our findings is unclear, the biological significance of SAH-mediated detrimental effect, secondary to elevated intracellular Hcy, is an interesting area awaiting further exploration.

An endogenous proteinacious inhibitor in porcine liver for S-adenosyl-L-methionine dependent methylation reactions: identification as oligosaccharide-linked acyl carrier protein

A proteinacious inhibitor of S-adenosyl-L-methionine (AdoMet)-dependent transmethylation reactions was purified to homogeneity from porcine liver by size exclusion chromatography and FPLC. The molecular weight of the inhibitor was 12,222 Da. A 7400 Da polypeptide fragment of the purified inhibitor was sequenced by matrix-associated laser desorption ionization; time-of-flight MS, and was found to be identical with the known sequence of spinach acyl carrier protein (ACP). Although the remainder of the molecule was not clearly defined, 1H and H-H correlation of spectroscopy (COSY) NMR analysis revealed the presence of an oligosaccharide with alpha-glycosidic linkage. The purified oligosaccharide-linked ACP inhibited several AdoMet-dependent transmethylation reactions such as protein methylase I and II. S-farnesylcysteine O-methyltransferase, DNA methyltransferase and phospholipid methyltransferase. Protein methylase II was inhibited with a Ki value of 2.4 x 10(-3) M in a mixed inhibition pattern, whereas a well-known competitive product inhibitor S-adenosyl-L-homocysteine (AdoHcy) had Ki value of 6.3 x 10(-6) M. Commercially available active ACP fragments (65-74) and ACP from Escherichia coli had less inhibitory activity toward S-farnesylcysteine O-methyltransferase than the purified inhibitor. The biological significance of this oligosaccharide-linked ACP which has two seemingly unrelated functions (inhibitor for transmethylation and fatty acid biosynthesis) remains to be elucidated.

Studies on naturally occurring proteinous inhibitor for transmethylation reactions

An inhibitor for S-adenosyl-L-methionine (AdoMet)-dependent methyltransferases has been purified from rat liver particulate fraction to apparent homogeneity, as judged by high-performance liquid chromatography, two-dimensional paper electrophoresis and isoelectric focusing chromatography. This inhibitor molecule, which is composed of 27 amino acid residues with an additional fluorescent chromophore, is rich in glycine, contains no basic amino acid, and has an isoelectric point (pI) of 3.70. A single absorption peak was observed at 248 nm in acidic as well as in neutral media, while two peaks were detected in alkaline medium at 206 nm and 248 nm. The former peak was found to be quite labile. The fluorescent spectra with excitation peak at 285 nm and emission peak at 358 nm are greatly influenced by the pH, being the highest in alkaline medium. The purified inhibitor inhibits all the AdoMet-dependent methyltransferases examined.

Impairment of insulin release by methylation inhibitors

The possible participation of enzymatic methylation reactions in the process of insulin release was investigated in rat pancreatic islets. The combination of 3-deazaadenosine and DL-homocysteine impaired the incorporation of 3H-methyl from L-[methyl-3H]methionine into endogenous islet proteins and phospholipids, but failed to affect turnover in the phosphatidylinositol cycle. The inhibitors of methylation decreased insulin release evoked by D-glucose or the combinations of D-glucose and gliclazide, L-leucine and L-glutamine, or Ba2+ and theophylline. The inhibitors of methylation did not impair either the oxidation of D-glucose or affect its capacity to decrease K+ conductance, stimulate Ca2+ inflow and provoke 45Ca accumulation in pancreatic islets. It is proposed that, in the process of insulin secretion, a methyl acceptor protein and/or phospholipid play(s) a limited modulatory role in the coupling of cytosolic Ca2+ accumulation to exocytosis.

Methylation of phospholipids in microsomes of the rat aorta

The methylation of phospholipids by S-adenosyl-L-methionine was characterized in microsomes prepared from strips of rat aorta. In the presence of 0.5 microM S-adenosyl-L-methionine, endogenous phosphatidylethanolamine was methylated to form three products: phosphatidyl-N-monomethylethanolamine, phosphatidyl-N,N-dimethylethanolamine and phosphatidylcholine. In the presence of 150 microM S-adenosyl-L-methionine the methylation activity increased more than 50-fold and the principal radioactive product was phosphatidylcholine. Optimal activity was at pH 9 and no magnesium requirement was detected. Exogenous phosphatidylethanolamine, phosphatidyl-N-monomethylethanolamine and phosphatidyl-N,N-dimethylethanolamine served as substrates for the enzyme. The methylation of exogenous phosphatidyl-N,N-dimethylethanolamine proceeded at a slower rate. Incubation of trypsin with the aorta microsomes reduced the enzymatic activity and reduced the relative yield of phosphatidyl-N-monomethylethanolamine. Phospholipase C degraded the methylated phospholipids, but phosphatidyl-N,N-dimethylethanolamine appeared to be less accessible to the phospholipase. The phospholipid methylation activity was inhibited by the addition of S-adenosyl-L-homocysteine or by L-homocysteinethiolactone. When intact strips of rat aorta were incubated with L-[methyl-3H]methionine, [3H]methyl groups were incorporated into phospholipids. This incorporation was inhibited when L-homocysteinethiolactone was added to the incubation. Polarized fluorescence of diphenylhexatriene in aorta microsomes was measured to determine the apparent membrane fluidity. When intact strips of aorta were incubated with methionine or with L-homocysteinethiolactone, methionine enhanced and L-homocysteinethiolactone decreased apparent fluidity of the microsomal membranes. Phospholipid methylation activity was examined in aorta microsomes prepared from genetically spontaneous hypertensive SHR strain rats. Phospholipid methylation activity was substantially greater in the SHR aorta microsomes than in microsomes prepared from Wistar-Kyoto WKY control strain aorta. Membrane fluidity was greater in the SHR aorta microsomes than in the WKY aorta microsomes. The hypothesis that phospholipid methylation activity influences fluidity of membranes and the possible involvement of methylated phospholipids in aorta membrane functions are discussed.

S-adenosyl-L-homocysteine: simplified enzymatical preparation with high yield

S-adenosyl-L-homocysteine hydrolase catalyses the synthesis of S-adenosyl-homocysteine (AdoHcy) from adenosine (Ado) and L-homocysteine (L-Hcy) in the absence of other enzymes, such adenosine deaminase, using Ado or L-Hcy as a substrate. Studies concerning the effect of substrates and enzyme concentrations with enzyme from beef liver, an inexpensive enzyme source, allowed the determination of the smallest amount of enzyme leading to total conversion to AdoHcy for the highest Ado and L-Hcy concentrations. Washing out the blood, homogenization of beef liver slices at pH 4.9 and precipitation with between 40% and 50% saturated (NH4)2SO4 eliminated contamination of tissue by blood adenosine deaminase. This enzyme preparation transforms Ado only into AdoHcy in the presence of L-Hcy. In a 1 1 final volume, an amount of enzyme preparation corresponding to at least 18.5 mg of fresh liver causes the total transformation of 60 mM Ado in the presence of 90mM L-Hcy after 40 hours incubation at 37 degrees C and pH 6.8. After purification by two crystallizations, the AdoHcy yield, based upon the Ado incubated, is about 80% of theory (1.0 g of AdoHcy per g of fresh beef liver).

Microsomal phosphatidylethanolamine methyltransferase: inhibition by S-adenosylhomocysteine

Inhibition by S-adenosylhomocysteine (AdoHcy) of the three reactions of phosphatidylethanolamine methyltransferase which catalyzes the production of phosphatidylcholine from phosphatidylethanolamine in guinea pig and rat liver microsomes has been evaluated. Five of the six methylation reactions in these two species exhibit greater affinity for inhibitor, AdoHcy, than for substrate, S-adenosylmethionine (AdoMet). The Ki values for the rate-limiting reactions were 3.8 microM and 68 microM in rat and guinea pig livers, respectively. An AdoMet : AdoHcy ratio of 12 : 1 in developing liver was found to decline to a constant value in the adult of 5 : 1. The concentration of AdoHcy in rat and guinea pig liver increases markedly following death of the animal. A concomitant decrease in the AdoMet level was observed in guinea pig liver. A comparison of phosphatidylethanolamine methyltransferase activity with the hepatic concentrations of AdoMet and AdoHcy in mouse, rat, rabbit and guinea pig is presented. Regulation of the methylation pathway is discussed.

Decreased transmethylation of biogenic amines after in vivo elevation of brain S-adenosyl-l-homocysteine

The ability of S-adenosyl-L-homocysteine (AdoHcy) to inhibit biologic transmethylation reactions in vitro has led us to explore the possibility of pharmacologically manipulating AdoHcy levels in vivo and examining the consequences of these alterations on the transmethylation of some biogenic amines. Swiss-Webster mice were injected intraperitoneally with different doses of adenosine (Ado) and D, L-homocysteine thiolactone (Hcy) and were killed at various times thereafter. S-Adenosyl-methionine (AdoMet) and AdoHcy concentrations were determined by using a modified isotope dilution-ion exchange chromatography-high pressure liquid chromatography technique sensitive to less than 10 pmol. Increasing doses of Ado + Hcy (50-1000 mg/kg of each) produced a dose-related increase in blood, liver, and brain AdoHcy levels. At a dose level of 200 mg/kg Ado + Hcy, AdoHcy levels were markedly elevated, with minimal concomitant perturbations of AdoMet. This elevation was maximal 40 min after giving Ado + Hcy, returning to control values within 6 h. Ado + Hcy treatment resulted in decreased activities of catechol-O-methyltransferase, histamine-N-methyltransferase, and AdoHcy hydrolase in vitro. The cerebral catabolism of intraventricularly administered [(3)H]histamine (HA) was decreased in a dose-related manner by Ado + Hcy treatment as evidenced by higher amounts of nonutilized [(3)H]HA in brain, concurrent decreases in [(3)H]methylhistamine formation, and decreases in the transmethylation conversion index. Steady state levels of HA also showed dose-related increases after Ado + Hcy treatment. It is concluded that injections of Ado + Hcy can markedly elevate AdoHcy levels in vivo, which can, in turn, decrease the rate of transmethylation reactions.

Affinity chromatography of an S-adenosylmethionine-dependent methyltransferase using immobilized S-adenosylhomocysteine. Purification of the indolethylamine N-methyltransferases of phalaris tuberosa

In the cases that have been studied so far, S-adenosylhomocysteine (SAH) is a powerful inhibitor of S-adenosylmethionine (SAM) binding to SAM-dependent methyltransferases. We deduced, from the available data on the binding of SAM and SAH analogues to SAM dependent methyltransferases, that linkage of SAH through the carboxyl group to an immobilized support would lead to a more general affinity adsorbent for SAM-dependent methyltransferases than linkage through other functional groups. This paper describes the synthesis of this affinity adsorbent and its use to purify the two indolethylamine N-methyltransferases of Phalaris tuberosa.