Radioimmunoassay for neopterin in body fluids and tissues.

Specific antibodies against D-erythroneopterin have been prepared in rabbits using a conjugate of D-erythroneopterin to bovine serum albumin (D-erythroneopterinylcaproyl-bovine serum albumin). The antiserum distinguished D-erythroneopterin from other pteridines, i.e., three stereoisomers of neopterin, L-erythrobiopterin, folic acid, xanthopterin, and four other synthetic pteridines. Using this specific antiserum, a radioimmunoassay for D-erythroneopterin has been developed to measure the neopterin concentrations in urine and tissues. The conjugate of D-erythroneopterin with tyramine (NP-Tyra) was synthesized and labeled with 125I as the labeled ligand NP-[125I]tyra for the radioimmunoassay. The minimal detectable amount of neopterin was about 0.1 pmol. The concentration of total neopterin (neopterin, 7,8-dihydroneopterin, quinonoid dihydroneopterin, and tetrahydroneopterin) in the biological samples was obtained by iodine oxidation under acidic conditions prior to the radioimmunoassay, and that of neopterin plus 7,8-dihydroneopterin by oxidation under alkaline conditions. Total neopterin values in human urine obtained by this new radioimmunoassay showed a good agreement with those obtained by high-performance liquid chromatography with fluorescence detection. With rat tissue samples which contained very low concentrations of neopterin as compared to biopterin, biopterin was simultaneously determined by our previously reported radioimmunoassay, and neopterin values were corrected for the cross-reactivity (0.1%). The neopterin concentrations obtained by this method agreed with the values obtained by the radioimmunoassays for neopterin and biopterin after their separation by high-performance liquid chromatography. This very small amount of neopterin, as compared with biopterin, in rat tissues could not be determined by high-performance liquid chromatography-fluorometry alone due to the masking of the neopterin peak by a large biopterin peak.(ABSTRACT TRUNCATED AT 250 WORDS)