STUDIES OF MONOAMINE OXIDASES¹: PROPERTIES OF THE ENZYME IN BOVINE AND RABBIT BRAIN MITOCHONDRIA

Abstract

Brain mitochondria were prepared from rabbit and bovine cerebral cortex and the purity and intactness of the preparation assessed through the use of enzyme markers and electron microscopy. Enzymatic properties of monoamine oxidase were studied in the purified mitochondrial preparations which were essentially devoid of major contamination by other organelles, especially microsomes. Five substrates were used for characterization of the enzyme: dopamine, kynuramine, serotonin, tryptamine and tyramine. It was found that there was considerable substrate variation in the properties, but in general, the two species showed similar characteristics. The more pertinent findings were: (1) apparent Km values ranged from 1.1 ± 10−5m for tryptamine to 2.5 ± 10−4m for dopamine; (2) substrate specificity from Vmax values in decreasing order was tyramine > dopamine > kynuramine > serotonin > tryptamine for the bovine enzyme and tyramine > kynuramine > dopamine > serotonin > tryptamine for rabbit; (3) there appeared to be three distinct pH optima according to substrate: pH 7.5 for phenylethylamines, pH 8.2–8.5 for the indolylamines and pH 9.1 for kynuramine; and (4) the activity with tyramine was highly sensitive to increased oxygen tension while kynuramine showed no sensitivity. It is proposed that the properties of monoamine oxidase, a membrane‐bound enzyme, might be influenced by the microenvironment and results are also discussed in terms of multiple forms or multiple activity sites on a single form.

Sensitivity to vanadate and isoforms of subunits A and B distinguish the osteoclast proton pump from other vacuolar H+ ATPases.

“…(37). The different subcellularfractions were characterized by assaying specific marker enzymes (15)(16)(17)(18)(19). The results indicated a 6-to 8-fold enrichment in the P1 and a 9-to 11-fold enrichment in plasma membrane in the P2 fractions with low levels ofcontamination by other organelles ( Table 1).…”

In vitro INHIBITION OF BRAIN MITOCHONDRIAL MONOAMINE OXIDASE BY 6‐HYDROXYDOPAMINE

“…The deamination of dopamine was inhibited in a similar way by 6-OHDA, the Ki value being 176 MM which is close to the Km (160 MM) for dopamine (Achee, Togulga & Gabay, 1974 50 mM, some five hundred times greater than the enzyme Km (see Tipton, 1968;Tipton & Spires, 1971). …”

Some interrelated properties of A and B form monoamine oxidase in monkey brain mitochondria.

“…The activity of A-form MAO may be more dependent on the conditions of the reaction medium. These results indicate that the activity of A-form MAO is affected easily and may be inactivated under some con ditions, that is, by effects of pH (43), various anions and heat treatment (44). On the other hand, B-form MAO showed broad pH activity curves and high activity at wide pH ranges.…”