Abstract
Exposure to o-phthaldialdehyde (OPT) causes intense fluorescence in A2-cells in sections from pancreas. Alloxan treatment of mice was followed by a marked increase in the number and fluorescence intensity of the OPT-reactive A2-cells, whereas cobalt chloride given to guinea pigs induced an almost total depletion of the cytoplasmic fluorescence with concomitant reduction of their argyrophilia. Microspectrofluorometric analysis of the fluorophore revealed excitation maxima at 370 and 420 mµ; the corresponding emission maxima were 430 and 490 mµ. This closely resembled the spectral characteristics of the OPT-induced fluorescence of authentic glucagon. It is concluded that OPT forms fluorphores wth glucagon in the A2-cells and that the NH2-terminal histidine residue of the polypeptide is responsible for the formation of the fluorophore. It is proposed that OPT is a useful reagent for the histochemical detection not only of histamine but also of histidyl peptides.