Acrylodan can label amino as well as sulfhydryl groups: results with low-density lipoprotein, lipoprotein[a], and lipid-free proteins.

Human plasma lipoprotein[a] and autologous low-density lipoprotein were reacted with the fluorescent probe 6-acryloyl-2-(dimethylamino)naphthalene (acrylodan) previously reported to be specific for sulfhydryl groups. Reaction kinetics were biphasic in both cases. The reaction of bovine serum albumin with acrylodan was also biphasic. Monophasic kinetics were observed when protein free sulfhydryl groups were blocked by carboxamidomethylation prior to acrylodan reaction. A significant increase in total fluorescence was observed in the reaction of acrylodan with proteins containing no free sulfhydryl groups and with polylysine. The rates of these reactions were highly sensitive to pH. Fluorescence changes due to dissolution of probe into hydrophobic protein or lipid domains were minimal as was reaction of probe with phospholipid head groups. When isolated from acrylodan-labeled Lp[a], apo[a], which contains no free sulfhydryl groups, contained covalently bound acrylodan. These results suggest that acrylodan can modify the lysine residues of lipid-free proteins and may modify not only the free sulfhydryl groups of low-density lipoprotein and lipoprotein[a] but also reactive amino groups. We conclude that under these conditions, the use of this probe to quantify free sulfhydryl groups in these lipoproteins is infeasible.