ELISA analysis of endogenous proBNP and BNP using GTX04088 BNP antibody [24C5]. Gel filtration studies of endogenous proBNP and BNP. Proteins from one representative plasma sample of a HF patient were separated on the Superdex Peptide column (GE Healthcare). BNP immunoreactivity in fractions was measured by the 50E1-24C5 assay. The first peak corresponds to proBNP, while the second peak corresponds to the BNP form.