Metabolism and axonal transport of polyamines in a single identified neuron of Aplysia californica

Abstract

The metabolism of polyamines was investigated by injecting purified [3H]putrescine directly into the soma of the giant neuron R2 of Aplysia. Injected putrescine was rapidly metabolized to spermidine, spermine, and several catabolites, including GABA and monoacetylputrescine. Identification of these products was by comparison with the authentic compound using ion exchange chromatography. When R2 was injected with amounts of [3H]putrescine determined so that the intracellular content of labeled precursor was less than 6 X 10(-6) M, metabolism was rapid and occurred via pathways similar to those in mammalian tissues. At concentrations of labeled precursor greater than 2 X 10(-4) M, relatively little putrescine was converted to product. By 4 h after injection, putrescine and its labeled products appeared in R2's axon, where additional metabolism occurred. These results indicated that the enzymes involved in polyamine interconversion are not restricted to R2's cell body, and this suggestion was corroborated by finding ornithine decarboxylase and S-adenosylmethionine decarboxylase activities in Aplysia nerves. The distribution of the polyamines along R2's axon was compared with that of 3H-glycoproteins, with the finding that while the acid-soluble polyamines move by diffusion, labeled polyamines associated with protein are rapidly transported.