Type A and B monoamine oxidase activities in the human and rat kidney

Abstract

The present work has examined the distribution of the two isoforms of monoamine oxidase (MAO), type MAO‐A and MAO‐B, in the cortex and medulla of the human and rat kidney. Homogenates of renal cortex and renal medulla were prepared in 67 mmoles 1^‐1phosphate buffer (pH = 7.2) and MAO activity was determined with [³H]5‐hydroxytryptamine ([³H]5HT) and [¹⁴C]β‐phenylethylamine ([¹⁴C]β‐PEA) as preferential substrates of type A and type B MAO, respectively. K_m and V_max values for the two substrates were also calculated. Both MAO‐A and MAO‐B are present in the cortex and the medulla of the human and rat kidneys. In the human kidney, MAO‐A activity was found to be similar in the cortex (V_max= 142.70±45.05 nmoles mg^‐1protein h^‐1) and medulla (V_max= 133.91±35.51 nmoles mg^‐1protein h^‐1); MAO‐B activity was found to be higher in the cortex (V_max= 166.19±19.75 nmoles mg¹protein h^‐1) than in the medulla (V_max= 92.91±13.22 nmoles mg^‐1protein h^‐1). In the rat kidney, MAO‐A was also found to be similar in the cortex (V_max= 62.35±1.74 nmoles mg^‐1protein h^‐1) and the medulla (V_max= 59.42±0.97 nmoles mg^‐1protein h^‐1) and higher than the activity of MAO‐B in the two renal areas (cortex, V_max= 31.06±1.09 nmoles mg^‐1protein h^‐1; medulla, V_max= 14.93±0.97 nmoles mg^‐1protein h^‐1). No statistically significant differences were found between the K_m values towards [³H]5HT and [¹⁴C]β‐PEA in the cortex and the medulla of the human and rat kidneys. The results show that in both renal areas, activity of the enzyme is higher in the human kidney than in the rat kidney. Furthermore, in the human kidney, in contrast with the rat kidney, MAO‐B activity closely follows MAO‐A activity.