Dietary and systemic phenylalanine utilization for mucosal and hepatic constitutive protein synthesis in pigs

The objective of this study was to quantify the utilization of dietary and systemic phenylalanine for mucosal and hepatic constitutive protein synthes is in piglets. Seven female piglets (7.6 kg) bearing arterial, portal? peri pheral venous, and gastric catheters were fed a high-protein diet and infus ed intragastrically with U-C-13-labeled protein and intravenously with [H-2 (phenyl)(5)]phenylalanine ([H-2(5)]phenylalanine) for 6 h. The isotopic enr ichment of the two phenylalanine tracers was measured in arterial and porta l blood, in mucosal and hepatic-free and protein-bound phenylalanine, and i n very low-density apolipoprotein B-100, albumin, and fibrinogen. The relat ive isotopic enrichments of the tracers in mucosal-free (ratio of H-2(5)- t o U-C-13-labeled = 0.20 +/- 0.05) and protein-bound (0.32 +/- 0.08) phenyla lanine differed significantly (P < 0.01). Although this suggests preferenti al use of arterial phenylalanine for mucosal protein synthesis, on a molar basis, 59 +/- 6% of the mucosal protein was derived from dietary phenylalan ine. There were significant differences (P < 0.025) between the relative la beling of the two tracers in arterial (ratio of H-2(5)- to U-C-13-labeled = 1.25 +/- 0.48) and portal(ratio of H-2(5)- to U-C-13-labeled = 0.72 +/- 0. 18) phenylalanine. The mean ratio of the two tracers in all proteins of hep atic origin that were analyzed (0.69 +/- 0.18) was similar to that of porta l phenylalanine. We conclude that in the fed state portal phenylalanine is preferentially used for constitutive as well as secreted hepatic protein sy nthesis.