295 Synthesis of glycine from trans-4-hydroxy-L-proline in tissues of juvenile hybrid striped bass (<i>Morone chrysops</i> x <i>M. Saxatilis</i>)

Abstract Trans-4-Hydroxy-L-Proline (Hyp) is a major amino acid in the collagen of fish. The turnover of collagen in various tissues releases a large amount of Hyp, which has traditionally been considered as a metabolic waste. However, results of recent studies have identified the conversion of Hyp into glycine in piglets. The present study tests this hypothesis that hybrid striped bass (HSB, a carnivorous fish species) can form glycine from Hyp. Fish were fed a standard diet containing 60% fishmeal for 10 wk and grew from 5.5 to 45 g during a 9-wk period. Slices (~75 mg) of skeletal muscle, liver, proximal intestine, gills, and brain were obtained from 45-g HSB (n = 6) and incubated at 26 °C for 2 h in 1 mL of oxygenated (95% O2/5% CO2) Krebs-Henseleit bicarbonate buffer (pH 7.4, with 5 mM D-glucose) containing 0, 0.2, or 2 mM Hyp. After a period of 2 h incubation, 200 μL of 1.5 M HClO4 was added into the incubation medium to stop the reaction. The acidified medium plus tissue together was homogenized and the homogenizer was rinsed with 1 mL of water, followed by the addition of 100 μL of 2 M K2CO3. The neutralized extract was analyzed for amino acids using HPLC involving o-phthaldialdehyde derivatization. Tissues without incubation were also determined for amino acids. Data were statistically analyzed by a paired t-test. Glycine was produced from 0.2 and 2 mM Hyp by the liver and skeletal muscle, and 2 mM Hyp by the intestine (Table 1). Glycine was further converted into serine in the liver, but not skeletal muscle or intestine. The production of glycine or serine from Hyp was not due to net protein breakdown in tissues. Tyrosine was measured as an indicator of protein degradation. There was no detectable generation of glycine from 0.2 or 2 mM Hyp by the gills or brain of HSB. These results indicate the synthesis of glycine from Hyp by juvenile HSB in a tissue-specific manner. Because skeletal muscle accounts for ~45% of the body weight, this organ is the major site for this metabolic pathway in the fish. (Supported by USDA/NIFA grant No. 2022-67015-36200)