Effect of growth hormone on polyamine synthesis in rat liver

Murphy, Brian Joseph Patrick (1975) Effect of growth hormone on polyamine synthesis in rat liver. Masters thesis, Memorial University of Newfoundland.

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Abstract

One of the initial events in the response to exogenous growth hormone in rats is a marked increase in the activity of hepatic ornithine decarboxylase, the rate limiting enzyme for polyamine biosynthesis. It is thought that polyamines could participate in the control of protein synthesis, both at the level of transcription and translation. Therefore, it would appear probable that their synthesis could be an important step in the protein anabolic response of the liver to growth hormone. – Ornithine decarboxylase activity is generally assayed by the release of ¹⁴CO₂ from ornithine 1-¹⁴c but this carbon may also be released by the sequential action of the mitochondrial enzymes ornithine transaminase, ∆¹ pyrroline-5-carboxylate dehydrogenase, glutamate dehydrogenase and α-ketoglutarate dehydrogenase. Therefore, the assay of ornithine decarboxylase in crude homogenate requires inhibition of this pathway. In the present study, aminooxyacetate (10-5M) inhibited over 98% of ornithine transaminase activity and virtually eliminated decarboxylation of ornithine by mitochondria, without any effect on cytoplasmic ODG activity. That the cytoplasmic activity represented true ODC was shown by the stoichiometric production of CO₂ and putrescine from ornithine. -- Subcellular fractionation of rat liver by differential centrifugation followed by assay of ornithine decarboxylase in presence of aminooxy-acetate and of putrescine dependent S-adenosyl methionine decarboxylase, ornithine transaminase and of marker enzymes from each fraction, demonstrated that ornithine decarboxylase and putrescine dependent S-adenosyl methionine decarboxylase are exclusively located in the cytosol, while ornithine transaminase is exclusively located in the mitochondrial fraction. The greatly increased ornithine decarboxylase activity observed after growth hormone administration was also found to be localized entireIy in the cytoplasm. The specific activities of putrescine dependent S-adenosyl methionine decarboxylase and ornithine transaminase showed no change 4 hours after growth hormone administration; their sub-cellular location also remained unaffected. -- Km values for hepatic ornithine decarboxylase using ornithine as a substrate showed no significant differences between control and growth hormone treated rats. Both displayed Km values of about 32 μM. The dissociation constants for putrescine as activator of putrescine dependent S-adenosyl methionine decarboxylase were also unchanged 4 hours after growth hormone injection (270 μM). -- Ornithine decarboxylase showed a maximal specific activity in liver cytosol at weaning in control rats and then decreased rapidly to a low level of activity about one week later. At all ages tested, growth hormone injection four hours prior to sacrifice resulted in an elevation of ornithine decarboxylase activity. However, this response was most marked in young rats (about weaning age) and in older rats (greater than five weeks of age). For reasons as yet not evident, the response in three to five week old male rats was greatly attenuated. -- A steady increase in activity of ornithine transaminase was observed from neonates to one month of age. This activity was maintained into adulthood and then began to decline steadily in the older rats. There was no change in ornithine transaminase activity four hours after growth hormone administration at any of the ages tested. -- Putrescine concentration in rat liver showed a progressive decline with age. A single injection of growth hormone (2 mg/100 g body weight) four hours before sacrifice resulted in an approximate doubling of putrescine concentration. Spermidine concentrations also declined with age, but were unaffected by growth hormone. The concentration of spermine increased to a maximal level at five weeks of age and then steadily declined. Growth hormone was also without effect on the concentration of this polyamine four hours after injection.

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