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.

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Pollen from fertile plants of corn (Zea mays L.), with normal and restored male‐sterile cytoplasm (cms), from various sources was assayed for in vitro germination on media containing Helminthosporium maydis pathotoxins extracted from infected leaves. The “pathotoxin” from H. maydis race O inhibited neither germination nor growth of pollen from any of the cms or normal cytoplasm sources tested, as compared with controls. The pathotoxin from H. maydis race T, however, inhibited in vitro germination and growth of pollen from cms‐T and cms‐P plants. Growth of pollen from cms‐C, cms‐S and normal cytoplasm sources was not inhibited by race T pathotoxin. The advantages of a pollen bioassay are discussed, as are possible special applications of this procedure.

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