The C-3 Epimer of 25-Hydroxyvitamin D3 Is Present in Adult Serum

Context: Epimers have identical molecular structure but differ in stereochemical configuration. It is widely believed that the C-3 epimer of 25-hydroxyvitamin D-3 [3-epi-25(OH)D-3] is found only in neonates. However, this epimer was recently detected in a limited number of adults. The physiological importance of 3-epi-25(OH)D-3 is uncertain but might affect 25-hydroxyvitamin D test results and thereby reliability of the 25-hydroxyvitamin D-3 [25(OH)D-3] measurement. Objective: This project describes development of a highly sensitive method for 3-epi-25(OH)D-3 measurement and establishes the prevalence of this epimer in adult clinical serum specimens. Design, Setting, Participants, and Main Outcome Measure: Serum 25(OH)D-3, 3-epi-25(OH)D-3, and 25(OH)D-2 concentrations were determined in a cohort of patients (n = 214; age neonate to 80 + yr). High-performance liquid chromatography with ultraviolet detection and high-performance liquid chromatography tandem mass spectrometry with atmospheric pressure chemical ionization equipped with cyanopropyl analytical columns were used to baseline separate and quantitate 25(OH)D-3, 3 epi-25(OH)D-3, and 25(OH)D-2. Results: The C-3 epimer was detected in 212 of 214 (99%) of samples. Concentrations ranged from 1 to 93 ng/ml for 25(OH)D-3 and 0.1 to 23.7 ng/ml for 3-epi-25(OH)D-3. The relative amounts of epimer to 25(OH)D-3 ranged from 0 to 25.5% (mean 4.75%). The epimer amount increased as 25(OH)D-3 increased in a nonlinear mode. In sera with approximately the same 25(OH)D-3 concentration, the ratio of epimer to 25(OH)D-3 varied, e.g. at 25(OH)D-3 values of 20-22 ng/ml, the ratio varied from 2-8.5%. Conclusion: 3-Epi-25(OH)D-3 is present in the majority of human serum specimens. Although this concentration is generally low, further work must investigate the impact of 3-epi-25(OH) D-3 on the various 25-hydroxyvitamin D assays and ultimately what information, if any, C-3 epimer measurement can provide clinically. (J Clin Endocrinol Metab 97: 163-168, 2012)