A general method to measure the substrate specificity of either a single or a series of enzymatically catalyzed reactions is described. This measurement is calculated as a discrimination ratio and is determined by four time dependent functions describing the ratio of incorporation rate of normal to analogue amino acids into specific proteins. Discrimination ratios for the synthesis of ribosomal proteins as a function of molecular weight and for the transmethylation of ribosomal RNA are determined as a function of age in female C57BL/6J mouse liver. Methionine and ethionine are used as the normal and analogue amino acid pair. Different ribosomal proteins from the same age animal had different discrimination ratios, generally increasing with increasing molecular weight (from 20 at 11,000 daltons to 150 at 52,000 daltons). A general decrease in discrimination ratio was found of age (115 to 594 days), ranging from 10 to 50%—depending on the molecular weight of the ribosomal protein. Transmethylation of ribosomal RNA also showed a slight decrease in discrimination ratio over the same age interval. These results are compared to amino acid analogue studies using in vitro aged human tissue culture cells.