HMOX1 and obesity due to melanocortin 4 receptor deficiency: To further address the hypothesis that HO-1 mediates the anti-obesity and insulin sensitizing actions of MnTBAP, we next tested whether blockade of HO-1 activity with tin mesoporphyrin (SnMP) (17), a potent HO-1 inhibitor, would prevent the ability of MnTBAP to limit obesity and improve insulin resistance.