Menaquinone-4, but not vitamin K
, enhances testosterone production in I-10 cells. I-10 cells were treated with 10 μM vitamin K1 (K1), or menaquinone-4 (MK-4) for 24 h, and testosterone levels in the culture medium were determined by ELISA (A), or I-10 cells were treated with K1 or MK-4 for 3 h, and vitamin K levels in cells were determined by fluorescent-HPLC (B). Data are represented as means ± SE (n = 3). Values with different letters are significantly different at p < 0.05. ND, not detected.