Histochemical demonstration of primary amino groups with 2-hydroxy-1-naphthaldehyde (HNA): optimization of the method

2-hydroxy-1-naphthaldehyde (HNA) was used for the histochemical demonstration of primary amino groups. The Schiff-bases formed exhibited a double-banded maximum absorption at lambda = 420 nm and lambda = 400 nm, respectively. The dependence of the equilibrium of the Schiff-base formation upon the concentration of HNA, the reaction medium, especially upon its pH-value, was investigated with Ehrlich ascites tumour cells and rat liver parenchymal cells. Optimum conditions were elaborated for the histochemical reaction running either in absolute ethanol (HNA-EtOH) or in a mixture of ethanol and acetate buffer pH = 4 (HNA-pH4), the equilibrium of the reactions adjusted after 10 d and 1 d, respectively. The HNA-pH4-method can be used for the histochemical demonstration of primary alpha-, delta-, and epsilon-amino groups of proteins. The HNA-EtOH-method comprises primary amino groups of proteins and of other substances insoluble in absolute ethanol. Both histochemical methods proved to be reproducible.