Fetal hemoglobin of transfused neonates and spectrophotometric measurements of oxyhemoglobin and carboxyhemoglobin

In vitro inhibition of heme oxygenase isoenzymes by metalloporphyrins

OBJECTIVE

Neonatal jaundice results from an increased bilirubin production and decreased hepatic bilirubin conjugation and excretion. Severe hyperbilirubinemia is currently treated with phototherapy or exchange transfusion; however, its prevention by inhibiting bilirubin formation is a more logical strategy. Heme oxygenase (HO), with inducible (HO-1) and constitutive (HO-2) isoenzymes, is the rate-limiting enzyme in heme catabolism, producing equimolar amounts of bilirubin and carbon monoxide (CO). Metalloporphyrins (Mps) are heme derivatives that competitively inhibit HO and thereby suppress hyperbilirubinemia. No systematic studies have been reported evaluating whether the HO isoenzymes are inhibited differentially by various Mps. Identification of Mps that selectively inhibit the inducible HO-1 without affecting the 'housekeeping' HO-2 isoenzyme might be desirable in the clinical setting of hemolytic disease, in which the Hmox1 gene is greatly induced. Although bilirubin production is due to the activity of both HO-1 and HO-2, the inhibition of HO-1 with a relative sparing of HO-2 activity might provide the most selective approach for the treatment of hemolytic disease.

STUDY DESIGN

We determined for the deutero-, proto-, meso- and bis-glycol porphyrins with zinc, tin and chromium as central atoms, respectively, the concentration needed for 50% inhibition (I(50)) of HO-1 and HO-2 activities in rat spleen and brain tissue.

RESULT

For a given Mp, HO-1 activity was less inhibited than that of HO-2. The order of inhibitor potency of each Mp was nearly identical for both isoenzymes. Tin mesoporphyrin was the most potent inhibitor for both isoenzymes. HO-2 selectivity was greatest for tin protoporphyrin. Conversely, the Zn compounds were least inhibitory toward HO-2. No Mp preferentially inhibited HO-1.

CONCLUSION

Mps that produce a less inhibitory effect on HO-2, while limiting the response of the inducible HO-1, such as ZnPP, may be a useful clinical tool.

A simple method for measuring fetal hemoglobin by co-oximetry

A simple, rapid method is presented for assessing the fetal hemoglobin fraction (%HbF). The principle of the measurement is straightforward. If two co-oximeters differ in the extent to which fetal hemoglobin interferes with their measurements of a particular analyte and if no other source of interference is present, then the magnitude of the difference in the HbF-induced interference between the two instruments can provide a quantitative measure of the fetal hemoglobin fraction. The present method does not require measuring optical absorbances at specific wavelengths or solving multiple simultaneous equations. On the contrary, the present method requires only one measurement on each instrument and a single simple calculation that yields a useful estimation of %HbF. Finally, it should be noted that this method can also be used for quantifying the concentrations of other compounds if two co-oximeters are available that differ appreciably in the extent to which the compound of interest interferes with their measurements of a particular analyte.