Amine oxidase activities in brown adipose tissue of the rat: identification of semicarbazide-sensitive (clorgyline-resistant) activity at the fat cell membrane

Abstract

Abstract Amine oxidase activity, previously described in homogenates of brown adipose tissue of the rat, has now been investigated in preparations of isolated fat cells. It was found that the specific activities of both monoamine oxidase A (MAO) and of the semicarbazide-sensitive clorgyline-resistant amine oxidase (SSAO) were higher in isolated fat cells than in the original whole tissue. Brown adipocytes therefore represent a major source of both these enzymes. In plasma membranes prepared from these isolated brown fat cells by borate extraction there was a similar enrichment of activity of SSAO and of the plasma membrane marker enzyme, phosphodiesterase I. However in preparations of cell membranes made by binding the cells to polycation-coated beads, enrichment of phosphodiesterase I activity was much greater than that of SSAO. It is suggested that the disposition of the enzyme within the cell membrane may account for the discrepancy in these results, i.e. the sidedness of the membrane may be important. Histochemical visualization of enzyme activity in whole tissue at the ultrastructural level was undertaken. Positive staining of mitochondria was achieved in the presence of the MAO substrate, tryptamine. Staining around the edges of the brown fat cells was observed with the SSAO substrates, tyramine and benzylamine. Staining was largely absent when substrate was omitted or after pretreatment with the irreversible SSAO inhibitor, hydralazine and the slowly reversible inhibitor, semicarbazide. It is not definitely proven that this staining represents sites of enzyme activity but the results are consistent with evidence from other studies indicating that SSAO in brown adipose tissue of the rat may be found predominantly at the fat cell surface. The significance of these findings in relation to the possible function of SSAO is discussed.