Plasma semicarbazide-sensitive amine oxidase activity is elevated in diabetes mellitus and correlates with glycosylated haemoglobin.

1. Semicarbazide-sensitive amine oxidase is a common name for a group of heterogeneous amine oxidases which are present in various mammalian tissues, especially in vascular smooth muscle cells, cartilage and adipose tissue, but also in plasma. 2. Plasma semicarbazide-sensitive amine oxidase activity was elevated in a group of 104 patients with insulin-dependent diabetes mellitus compared with normal control subjects (555 +/- 172 versus 352 +/- 102 m-units/l, P < 0.0005). 3. Plasma semicarbazide-sensitive amine oxidase activity was higher in subgroups with either retinopathy or nephropathy or both [583 +/- 116 (n = 34), 581 +/- 229 (n = 10) and 646 +/- 249 m-units/l (n = 19), respectively] than in the subgroup without overt complications [486 +/- 129 m-units/l (n = 41), P < 0.005]. 4. Plasma semicarbazide-sensitive amine oxidase activity was positively correlated with plasma glycosylated haemoglobin (r = 0.40; P < 0.0001) and with log urinary albumin excretion (r = 0.26; P < 0.025). 5. The possibility that semicarbazide-sensitive amine oxidase, by its conversion of endogenous amines like methylamine and aminoacetone into cytotoxic aldehydes, plays a role in the development of microvascular complications in diabetes mellitus, needs further investigation.