A simple assay for measurement of urinary p-aminobenzoic acid in the oral pancreatic function test

A simple method for the measurement of p-aminobenzoic acid (PABA) in urine following oral administration of N-benzoyl- l-tyrosyl- p-aminobenzoic acid (Bz-ty-PABA), a synthetic peptide used for the assessment of exocrine pancreatic function, was investigated using p-dimethylamino cinnamaldehyde (DACA) as a chromogenic reagent. DACA reacted with PABA in acidic solution and the red dye which developed had an absorption spectrum with maximum at 550 nm. Urine containing PABA and its conjugated metabolites was hydrolyzed with HCl, DACA solution was added to a portion of the hydrolysate, and the color intensity was determined. When Bz-ty-PABA was administered orally to healthy humans and patients with pancreatic insufficiency, urinary PABA content determined by the DACA method was in good agreement with that by the diazo-coupling method. This DACA method is preferable to the diazo-coupling method with respect to the simplicity and rapidity of the procedure and to the stability of the reagent solution. It is especially useful for the above application in the clinical laboratory.