Inactivation of liver S-adenosylhomocysteine hydrolase in vitro of rats treated with erythro-9-(2-hydroxynon-3-yl)adenine

Oxygen partial pressure and free intracellular adenosine of isolated cardiomyocytes

Adenosine formation by the heart is known to critically depend on the ratio of oxygen supply to oxygen demand, but the sensitivity of cardiomyocytes to defined changes in PO2 is not known. Isolated metabolically stable rat cardiomyocytes were incubated up to 45 min at constant PO2 values ranging from 0.1 to 100 mmHg using a feedback-controlled incubation system (oxystat system). Changes of the free intracellular adenosine concentration were measured after trapping of adenosine by cytosolic S-adenosylhomocysteine (SAH) hydrolase in the presence of 200 microM L-homocysteine thiolactone. Rate of SAH formation was constant at a PO2 between 3 and 100 mmHg and gradually increased at PO2 less than 3 mmHg. Cellular ATP decreased only at PO2 less than 1 mmHg, and this was accompanied by a decline of oxygen consumption. Treatment of cells with 5.5 mM deoxyglucose and 4 micrograms/ml oligomycin increased SAH formation 60-fold and was associated with elevated intra- and to a lesser extent extracellular adenosine levels. Inhibition of nucleoside transport with 20 microM S-(p-nitrobenzyl)-6-thioinosine steepened the transmembrane adenosine gradient. Our findings suggest that the cardiomyocyte responds to metabolic poisoning and oxygen deprivation with an enhanced formation of adenosine. This adenosine is mainly formed intracellularly and reaches the extracellular space by diffusion. Threshold for adenosine formation is as low as 3 mmHg.

The potential use of mechanism-based enzyme inactivators in medicine

Mechanism-based enzyme inactivator, alanine racemase, S-adenosylhomocysteine hydrolase, D-amino acid aminotransferase, gamma-aminobutyric acid aminotransferase, arginine decarboxylase, aromatase, L-aromatic amino acid decarboxylase, dihydrofolate reductase, dihydroorotate dehydrogenase DNA polymerase I, dopamine beta-hydroxylase, histidine decarboxylase, beta-lactamase, monoamine oxidase, ornithine decarboxylase, serine proteases, testosterone 5 alpha-reductase, thymidylate synthetase, xanthine oxidase.

Inactivation of S-adenosylhomocysteine hydrolase by nucleosides

The irreversible inactivation of S-adenosylhomocysteine hydrolase purified from hamster and bovine liver by adenosine analogs substituted in the 5' and 2 positions has been investigated in detail. 5'-Cyano-5'-deoxyadenosine inactivates as potently as 9-beta-D-arabinofuranosyladenine (Ara-A). Substitution of the Ara-A at the 2 position by halogens or deleting N at the 3 position decreases its potency. Although weak, 2',3'-dideoxyadenosine can also inactivate the enzyme. The irreversible inactivation of the hydrolase in rat hepatocytes incubated with 2-chloroadenosine or 3-deaza-Ara-A could be demonstrated, concomitant with increases in 35S-labeled S-adenosylhomocysteine and S-adenosylmethionine in the hepatocytes.