Abstract
This study examined the absorption rate of D-phenylalanine (Phe) and L-Phe across the small intestine of rats, and the influence of intestinal wall cells and microbes on the degree of conversion of D-Phe to L-Phe using the stable isotope tracer technique. D-Phenyl (d5) alanine (D-Phe (d5)) was prepared by the hydrogendeuterium exchange method, and mixed in equimolar proportions with commercial L-Phe (d8). The mixture was administered to two groups of rats by intraperitoneal injection (I.P.) and oral ingestion (Or.), respectively. 1) Blood Phe (d5) and Phe (d8) levels were determined by the GC-MS-SIM method. (a) The concentration curve after I.P. showed that the half-life of the D-form (Phe (d5)) was 30min and that of the L-form (Phe (d8)) was 15min. (b) The curve after Or. suggested that the D-form was absorbed more slowly than the L-form. 2) L-Phe (d5) /D-Phe (d5) ratios in the blood were determined by the LC-MS-SIM method. L-forms converted from D-forms accounted for about 10% of the total available D-forms for both I.P. and Or. These results show that the participation of both intestinal microbes and the process of absorption through intestinal wall cells may be disregarded in the conversion of D-Phe to L-Phe.
