Selective Inhibitors of Membrane-Bound Semicarbazide-Sensitive Amine Oxidase (SSAO) Activity in Mammalian Tissues

Abstract

Semicarbazide-sensitive amine oxidase (SSAO, EC 1.4.3.6) is a group of enzymes highly sensitive to inhibition by semicarbazide. This high sensitivity distinguishes these enzymes from monoamine oxidase (MAO). Various mammalian tissues contain membrane-bound SSAO which metabolizes only the primary monoamines. Vascular and non-vascular smooth muscle cells have particularly high SSAO activity, but recently the enzyme activity has also been found in non-vascular smooth muscle cells. The substrate specificity of SSAO shows considerable species-related variations. A variety of compounds inhibiting MAO activity has also been identified as SSAO inhibitors. Among inhibitors, there is no specific SSAO inhibitor so far tested. Many studies reinforce the conclusion that inhibitory properties of some compounds against MAO activities has been markedly differed from their properties as SSAO inhibitors. 2-Bromoethylamine has been recently developed with a potent, selective and suicide SSAO inhibitor without any inhibitory effect on MAO activity Using this inhibitor, it is possible to study the role of the enzyme in mammalian tissues. As physiological role the increased concentrations of SSAO, especially in blood plasma, have been found in diabetic patients and experimental animals. This enzyme was found to be associated with translocation of the glucose transporter GLUT 4 into the adipose cell surface and involved in the signaling of glucose uptake. Recent studies showed that vascular SSAO metabolizes endogenous primary amines, allylamine, methylamine and aminoacetone, to the corresponding cytotoxic aldehydes. These aldehydes have been linked to the ability of diabetic complications such as neuropathy, retinopathy and nephropathy. Overproduction of such toxic aldehydes produced by increased SSAO activity was proposed to be potentially hazardous in diabetic complications. Thus, reduction or inhibition of SSAO may be beneficial in these pathological conditions. Clearly species-related differences in properties of SSAO must be taken into account in this respect, particularly when assessing if SSAO inhibition may have great application in human.