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Notes: Abstract: Tachykinin immunoreactivity has been quantified and characterized in extracts of bovine retinae by combining radioimmunoassay, gel permeation chromatography, and reverse-phase HPLC. Using an antiserum specific for the C-terminal hexapeptide amide of substance P, levels of 3.43 ± 0.33 ng g−1 and 12.45 ± 0.76 ng g−1 (mean ± SD, n = 5) were measured in extracts prepared by acidified ethanol and boiling 0.5 M acetic acid, respectively. Levels of neurokinin A immunoreactivity, assayed using an antiserum cross-reacting with neurokinin A (100%), neurokinin B (50%), neuropeptide K (85%), and substance P (〈0.1%) were 12.46 ± 0.47 ng g−1 and 7.20 ± 0.37 ng g−1 in the same extracts. Gel permeation chromatography identified a single substance P immunoreactant eluting with substance P standard, whereas two neurokinin A immunoreactants were resolved eluting with neuropeptide K and neurokinin A standards. Reverse-phase HPLC analysis resolved immunoreactivity eluting with substance P, neurokinin A, neuropeptide K, and neurokinin B and their respective methionine sulphoxides. The amount of immunoreactive material co-eluting with the respective sulphoxides was higher in acidified ethanol extracts, and sub stance P was most susceptible to oxidative modification. Subsequent incubation of synthetic substance P with dispersed bovine retinal cells resulted in rapid conversion to three metabolites identified and isolated by reverse-phase HPLC. Each had an amino acid composition identical to that of substance P, and the major product had the same retention time as substance P sulphoxide. Deamidation was ruled out on the basis of the unequivocal demonstration of both glutamine residues and a methionine amide residue on each metabolite. These data demonstrate the presence of the mammalian tachykinins substance P, neurokinin A, neurokinin B, and neuropeptide K in the bovine retina. The susceptibility of substance P to methionine oxidation has been confirmed by incubation with dispersed retinal cells, and this putative inactivation system may be of physiological relevance within the retina.