| Popis: |
Isosorbide dinitrate inactivated E1 and E2 isozymes of human aldehyde dehydrogenase (EC 1.2.1.3), abolishing both dehydrogenase and esterase activities. NAD promoted, whereas chloral and NAD protected the enzyme from inactivation. The inactivation was irreversible upon dialysis and occurred without incorporation of the 14C-labeled isosorbide dinitrate. Inactivation was associated with formation of products, isosorbide-2-mononitrate and isosorbide-5-mononitrate. At 25 degrees C there were two pathways of product formation: a fast pathway, sensitive to aldehyde dehydrogenase inhibitors, and a slower pathway insensitive to inhibitors. The fast product formation and inactivation occurred simultaneously, and both were inhibited by chloral and by the irreversible active site-directed inhibitor bromoacetophenone. At 0 degree C the slow product formation was abolished, allowing study of the enzyme catalyzed reaction. The inactivation of the E1 isozyme at 0 degree C occurred in a single turnover that accounted for 80% of catalytic activity loss with isosorbide-2-mononitrate being the major product. No nitrate was ever detected; at 25 degrees C, nitrite was detected but in less than stoichiometric amounts. The mononitrates were also substrates and inactivators of aldehyde dehydrogenase. Isosorbide-2-mononitrate had the lowest K(i) and k3 values for the E1 isozyme when compared with that of the other two nitrate esters of isosorbide. Reversibility of inactivation by 2-mercaptoethanol suggested involvement of enzyme sulfhydryls. The inactivation appears to be mechanism-based and involves the esterase function of aldehyde dehydrogenase. |
