The fate of proline in the African fruit beetle Pachnoda sinuata

Metabolic pathways of proline consumption in working flight muscles and its resynthesis were investigated in the African fruit beetle, Pachnoda sinuat a. Mitochondria isolated from flight muscles oxidise proline, pyruvate and alp ha-glycerophosphate, but not palmitoyl-carnitine. At low proline concentrat ions, the respiration rate during co-oxidation of proline and pyruvate is a dditive, while at high proline concentrations it is equal to the respiratio n rates of proline oxidation. Flight muscles have high activities of alanine aminotransferase and NAD(+)- dependent malic enzyme which are involved in proline metabolism. Glycogen p hosphorylase and glyceraldehyde-3-phosphate dehydrogenase (carbohydrate bre akdown) also display high activities, whilst 3-hydroxyacyl-CoA dehydrogenas e (fatty acid oxidation) showed low activity. During the oxidation of proline, mitochondria isolated from flight muscles produce equimolar amounts of alanine. The rates of oxygen consumption by th e mitochondria during this process lead to the conclusion that proline is p artially oxidised; This is confirmed by the incorporation of radiolabel fro m pre-injected [U-C-14] proline into alanine during a flight experiment wit h P. sinuata. Proline is resynthesised, in vitro, from alanine and acetyl-CoA in the fat body. High activities of enzymes catalysing such pathways (alanine aminotra nsferase, 3-hydroxyacyl-CoA dehydrogenase and NADP(+)-dependent malic enzym e) were found. The in vitro production of proline from alanine is equimolar suggesting that resynthesis of one proline molecule is accomplished from o ne alanine molecule and one acetyl-CoA molecule. One source of the acetyl-C oA for the in vitro synthesis of proline is the lipid stores of the fat bod y. Proline synthesis by fat body tissue is controlled by feedback. Alanine ami notransferase is competitively inhibited by high proline concentrations. (C ) 1999 Published by Elsevier Science Ltd. All rights reserved.