Determination of 1,25-dihydroxyvitamin D Concentrations in Human Colon Tissues and Matched Serum Samples

Background: A novel method to measure 1,25-dihydroxyvitamin D (1,25(OH)(2)D) in human tissue was developed and validated. The objectives of this study were to determine whether 1,25(OH)(2)D is present in human colon tissue and to characterize the relationship between human colon tissue and serum 1,25(OH)(2)D concentrations. Materials and Methods: Normal colon tissue specimens and matched serum samples were obtained from 30 patients who had undergone colectomy. Colon 1,25(OH)(2)D was measured by lipid extraction followed by enzyme immunoassay (EIA). Serum 1,25(OH)(2)D and 25-hydroxyvitamin D (25(OH)D) were measured by EIA. Vitamin D binding protein (DBP) was measured in a subset of serum and tissue samples. Results: Regression analysis indicated a significant positive correlation between serum and colon 1,25(OH)(2)D concentrations (r=0.58, p=0.0008). The corresponding intercept at zero serum 1,25(OH)2D was 21.5 pmol/kg (95% CI=16.95-25.98; p<0.001). Colon 1,25(OH)(2)D did not correlate significantly with serum 25(OH)D. DBP levels in tissue samples were negligible. Conclusion: The hormone 1,25(OH)(2)D can be successfully detected in human colon at physiologically relevant concentrations, partly determined by serum 1,25(OH)(2)D. The results support the notion of in vivo synthesis of 1,25(OH)(2)D within colon tissues.