A double antibody capture ELISA for the HO-1 protein has been developed to separately quantitate HO-1 protein. The use of 2.5% NP40 detergent greatly assists in freeing HO-1 protein from membranes and/or other cellular entities and increased the amount of HO-1 protein found in rat liver whole homogenates as well as the nuclear, mitochondrial and microsomal fractions. Use of the detergent NP40 did not substantially change HO-1 protein standard curves. The ELISA assay for HO-1 has been shown to be reproducible over (i) a 4-day trial period as well as (ii) almost 1 year of general laboratory use. Excellent specificity for the HO-1 isoform is shown by the failure of either the human HO-2 protein or HO-2 peptide (at concentrations as high as 1000 ng/ml) to generate any signal above background. At least a 300-fold greater signal comes from HO-1 protein as compared to the HO-2 protein. The EC50 is about 200 ng/ml for HO-1, and the minimum detectable level of the HO-1 protein is about 1 ng/ml. The ELISA assay for the HO-1 protein requires a total of 6 h to complete. Of the total cellular HO-1 protein, 20, 19, 9 and 3% appeared in the nuclear, microsomal, mitochondrial and high speed supernatant fractions, respectively. As expected, the highest concentration of HO-1 protein per total protein in a subcellular fraction was found in the microsomes. For many research projects utilizing this ELISA assay for HO-1 protein concentration, use of the whole homogenate will be an excellent choice, rather than use of the postmitochondrial or microsomal fractions. Much higher HO-1 protein levels were found in tissues of rats rather than mice. This may be because the capture antibody and secondary antibody were both raised against the rat and not the mouse forms of the HO-1 protein. In rats the HO-1 concentrations were 1067, 364, 194, 31, 28 19, 5 and 2 ng/g tissue in whole homogenates from testes, brain, liver, lung, spleen, kidney, small intestines and urinary bladder, respectively. The ELISA assay for HO-1 described here will be useful for HO-1 research studies in tissues and cell cultures of rats and mice. This ELISA for HO-1 may also work with human tissues and cells.
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