Measurement of Glucose, Insulin, and Amino Acids

Plasma glucose and insulin concentrations were measured using commercially available kits (GLUC3, Roche, Ref: 05,168,791 190, and Immunologic, Roche, Ref: 12,017,547 122, respectively). Plasma amino acid concentrations were quantified using UPLC-MS as described previously (1 (link)). Plasma phenylalanine concentrations and L-[ring-¹³C₆]-phenylalanine enrichments were determined by GC-MS (Agilent 7890A GC/5975C; MSD) as described previously (17 (link)). For measurement of muscle protein-bound L-[ring-¹³C₆]-phenylalanine enrichments, muscle protein was first extracted as described previously (2 (link)). Next, muscle protein-bound L-[ring-¹³C₆]-phenylalanine enrichments were measured using GC-isotope ratio MS (GC-IRMS) as described previously (17 (link)).