CYP24A1 codes for an enzyme that hydroxylates
25[(OH).sub.3][D.sub.3] or 1,25[(OH).sub.2][D.sub.3] to 24,25[(OH).sub.2][D.sub.3] or 1,24,25[(OH).sub.3][D.sub.3], less active forms of the hormone, and in that sense, might function as a double-edged sword.
If the external action of hydrolysis is weak, no [Ti.sup.4+] and Ti[(OH).sup.3+] component is represented, meaning that the capability of [Ti.sup.4+] is strong in generating various hydroxylates
. It is obvious because the constants ([pk.sub.1]~ [pk.sub.4]) of [Ti.sup.4+] hydrolysis reaction in different levels keep in very high value (13.06~14.15).
The prolyl hydroxylase 2 (PHD2) hydroxylates
the two proresidues that are present in the [O.sub.2]-dependent degradation domain of the HIF1[alpha], which primes HIF1[alpha] for degradation under normoxic conditions.
HIF-1[alpha] is expressed in all cells, has a half-life of ~1 min, and is regulated via 4-hydroxylation of proline residues (402 or 564) by PHDs, which is oxygen dependent and, in presence of sufficient oxygen, hydroxylates
these proline residues .
LH3 has three unique posttranslational modifications present in collagenous sequences in vivo and it hydroxylates
lysyl residues, galactosylates hydroxylysyl residues, and glucosylates galactosyl hydroxylysyl residues .
The Vitamin-K epoxide reductase (VKOR) enzyme hydroxylates
vitamin K and it converts it into active vitamin K (Vit-K(H2)).
Under normoxic conditions, prolyl hydroxylase domain protein 2 (PHD2), homologous to EGLN1, constitutively hydroxylates
specific residues on HIFl A, resulting in its degradation (Salceda and Caro, 1997; Epstein et al., 2001).
In the hypoxic stress pathway, factor inhibiting HIF (FIH) hydroxylates
HIF-1[alpha] but also has a range of ankyrin-repeat domain (ARD)-containing proteins as substrate (Cockman et al.
the key proline residues of HIF-1a, which targets HIF-1a for destruction via the ubiquitin-proteasome pathway, in normoxia.
Second, FXR induces the expression of genes that encode bile acid-modifying enzymes including CYP3A4 , sulfotransferase 2A1 (SULT2A1)  and UDP-glucuronosyltransferase 2B4 (UGT2B4) , which hydroxylates
, sulfidates, and glucuronidates bile acids to protect the liver from toxicity, respectively.
The liver enzyme CYP2C9 hydroxylates
THC at the C11 position, producing the equipotent metabolite, 11-hydroxy-THC (11-OH-THC) (23).