Identity of PSA purified from seminal fluid by different methods: Comparison by amino acid analysis and assigned extinction coefficients

To determine the true extinction coefficient of prostate specific antigen (PSA) and to measure any differences in PSA when isolated from seminal fluid by four different published methods, we studied 10 different lots of PSA by quantitative amino acid analysis. Despite an expected PSA concentration of 1 mg/ml based on gravimetric analysis at an average optical density of 1.45 at 280 nm, we recovered only 0.79 mg/ml by quantitative amino acid analysis (range 0.752 to 0.820 mg/ml with a coefficient of variation C.V. of 3.3% among the 10 lots). The concentration of 0.79 mg/ml was based on a molecular weight of 28,430 daltons for glycosylated PSA determined by ion spray mass spectroscopy Bélanger et al: Prostate, 27:187-197, 1995. From these 10 amino acid analyses, we calculated the extinction coefficient of PSA at 280 nm as 1.84 +/- 0.04 ml x mg-1 x cm-1 (range 1.78 to 1.90 with a C.V. of 2.2%). Similar concentrations of PSA were obtained by amino acid analysis regardless of the method of purification. These observations support the presence of a single form of PSA in seminal fluid and are consistent with the molecular evidence that PSA is transcribed from a single gene locus on the long arm of chromosome 19 Riegman et al.: Genomics 14:6-11, 1992. They do not support the recent contention by the Roswell Park group that the PSA they isolated in 1979 Wang et al.: Invest Urol 17:159-163, 1979; Wang et al.: Prostate 24:107-108, 1994 is different from p30 reported a year earlier Sensabaugh: J Forensic Sci 23:106-115, 1978.