Determination of aldehyde dehydrogenase isozyme activity in human liver

As acetaldehyde (Ac-CHO) has been implicated as a cause of alcoholic liver injury, accurate knowledge concerning changes in the Ac-CHO oxidizing system in human liver is essential for the understanding of the pathogenesis. However, an assay system for aldehyde dehydrogenase (ALDH: EC 1.2, 1.3) isozymes in human biological material has not yet been established. In the present study, the assay systems for human liver ALDH isozyme activity were analyzed. In human red blood cells, in which only one type of ALDH isozyme, high Km ALDH, is present, a maximum activity was observed at a substrate concentration of over 300 microM. In human liver of the usual type in which ALDH I (low Km isozyme) was not deficient, the activity reached a first plateau at 12 microM Ac-CHO after which the activity started to increase again at 20 microM Ac-CHO and continued to increase until 5.0 mM Ac-CHO. In the liver of the unusual type, which is deficient in low Km ALDH, activity was not detected at Ac-CHO concentrations lower than 10 microM. These results indicate that the optimum substrate concentrations for the determination of ALDH isozymes are 12 microM for low Km, 300 microM for high Km and over 1 mM for very high Km ALDH isozymes. The maximum activities of these three isozymes in the liver were obtained at a pH ranging between 9.0-9.5 and at an NAD concentration of over 500 microM. From these results, it is concluded that the assay system of Blair and Bodley is applicable for the determination of ALDH isozyme activity in human biological material with the exception of determining Km values.