MYOCARDIAL S-ADENOSYLHOMOCYSTEINE HYDROLASE IS IMPORTANT FOR ADENOSINE PRODUCTION DURING NORMOXIA

The coronary vasodilator adenosine can be formed in the heart by breakdown of AMP or S-adenosylhomocysteine (SAdoHcy). The purpose of this study was to get insight into the relative importance of these routes of adenosine formation in both the normoxic and the ischemic heart. A novel HPLC [high performance liquid chromatography] method was used to determine myocardial adenosine and SAdoHcy. Accumulation of SAdoHcy was induced in isolated rat hearts by perfusion with L-homocysteine thiolactone or L-homocysteine. The release of adenosine, inosine, hypoxanthine, xanthine and uric acid was determined. Additional in vitro experiments were performed to determine the kinetic parameters of S-adenosylhomocysteine hydrolase. During normoxia the thiolactone caused a concentration-dependent increase in SAdoHcy. At 2000 .mu.M of the thiolactone an SAdoHcy accumulation of 0.49 nmol/min per g wet wt was found during normoxia. L-Homocysteine (200 .mu.M) caused an increase of 0.37 and 4.17 nmol SAdoHcy/min per g wet wt during normoxia and ischemia, respectively. The adenosine concentration in ischemic hearts was significantly lower when homocysteine was infused (6.2 vs. 11.5 nmol/g; P < 0.05). Purine release was increased 4-fold during ischemia. The Km for hydrolysis of SAdoHcy was about 12 .mu.M. At in vitro conditions favoring near-maximal SAdoHcy synthesis (72 .mu.M adenosine, 1.8 mM homocysteine), the synthesis rate in homogenates was 10 nmol/min per g wet wt. From the combined in vitro and perfusion studies, it was concluded that S-adenosylhomocysteine hydrolase can contribute significantly to adenosine production in normoxic rat heart, but not during ischemia.