evidence for existence of type a mao in mitochondria from human placenta authors transl the existence of type a and b mao in mitochondria from human placenta was investigated on the basis of inhibitions by selective mao inhibitors such as clorgyline type a inhibitor and pargyline and deprenyl type b inhibitors with serotonin substrate for type a mao tyramine substrate for both types of mao and betaphenylethylamine substrate for type b mao as substrates and the results were compared with those obtained with mao in rat liver the rates of serotonin betaphenylethylamine and benzylamine oxidations by placental mao were approximately 191 12 and 48 to those of rat liver mao respectively placental mao was more sensitive to tryptic digestion than the enzyme in rat liver both maos could be separated into two fractions by sucrose density gradient centrifugation but the two types could not be distinguished when inhibitor sensitivity and substrate specificity experiments were carried out placental mao activity was inhibited by low concentrations of type a inhibitor and was relatively insensitive to those of type b simple sigmoidal and identical inhibition curves with various concentrations of either type a or type b inhibitors were obtained with these substrates these findings suggest that mitochondria mao in human placenta essentially consists of one distinguishable type of mao which closely resembles the type a mao found in other tissues of many species