HO-2

AcronymDefinition
HO-2Heme oxygenase type 2
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HO-1 (a) and HO-2 (b) expression (expressed as %) in the cardiac left ventricle (LV) of ovary-intact (black bar), ovariectomized (OVX (white bar)), and estrogen- (E2: (gray bar); 0.10 mg/kg/day, 2 weeks orally) or RAL-treated (RAL 0.33 (red bar): 0.33 mg/kg/day, RAL 1 (blue bar); 1.0 mg/kg/day, 2 weeks, orally) OVX rats.
HO-1 (a) and HO-2 (b) expression (expressed as %) in the aortic tissues of ovary-intact (black bar), ovariectomized (OVX (white bar)), and estrogen-treated ([E.sub.2]: (gray bar); 0.10 mg/kg/day, 2 weeks orally), or RAL-treated (RAL 0.33 (red bar): 0.33 mg/kg/day RAL 1 (blue bar); 1.0 mg/kg/day, 2 weeks, orally) OVX rats.
Primary sequence alignment shows that HO-1 and HO-2 exhibit two regions of sequence divergence: one is around residue 127 and the other is near the C-terminal between 240 and 295 (HO-2 numbering) (Figure 4) [66, 69].
The catalytic core of HO-1 and HO-2 includes a 24 amino acid segment in the primary structure (that corresponds to residues 146 to 169 in human HO-2 and 126 to 149 in HO-1) perfectly conserved among all forms [8].
Although the C-terminal HRM region shows the most significant sequence difference between HO-1 and HO-2, most of the key amino acids involved in heme binding, Lys 38, His 45, Glu 49, Tyr 154, Gly 159, Asp 160, Gly 163, Lys 199, and Arg 203 are observed in similar positions in the hemebound HO-2 structure when compared with heme-bound HO-1.
The Heme Regulatory Motifs in HO-2. A distinctive feature of HO-2 despite the high sequence identity and three-dimensional homology in the core domains between HO-1 and HO-2 is the presence of the HRMs in HO-2 described in 1997 [89].
HO-2 contains two conserved HRMs involving Cys 265 in HRM1 and Cys 282 in HRM2.
It has been established that HO-2 activity is substrate dependent and that factors that increase heme substrate availability also increase CO production [96].
HO-2 activity can also be regulated by the presence of NO and ROS.
A specific potent activator of HO-2 is menadione (Vitamin K3).
However, the use of [H.sub.2][O.sub.2] showed that HO-2H45A was also able to protect cells against oxidative stress injury, suggesting the multiplicity of action of the HO-2, besides its essential catalytic activity [105].
The main difficulty on HO activity regulation research and the specific contribution of HO-1 or HO-2, is the absence of specific inhibitors.