Detection of an advanced glycosylation product bound to protein in situ

Protein amino groups can react with glucose without the aid of enzymes to form stable Amadori products containing 1-amino-1-deoxyketose residues. These adducts can undergo subsequent rearrangements and dehydrations to form various brown and fluorescent pigments. Recently, a chromophore, 2-(2-furoyl)-4(5)-(2-furanyl)-1H-imidazole (FFI), was isolated from acid hydrolysates of bovine serum albumin (BSA) and poly-L-lysine which had been incubated with glucose. To confirm the presence of FFI in situ, a radioimmunoassay was developed. A derivative of FFI, 4-furanyl-2-furoyl-1H-imidazole-1-hexanoic acid, was coupled to BSA and used to immunize rabbits. A radioactive FFI derivative was synthesized by reaction of 2-furyl-glyoxal with gamma-amino-[2,3-3H]butyric acid to form FFI-[3H]butyric acid. The resultant antiserum showed binding affinity to FFI and cross-reactivity for related compounds. FFI bound to proteins was liberated by acid hydrolysis or digestion by proteinase K prior to measurement. A linear relationship was seen between the amount of FFI equivalent detected and the amount of acid hydrolysate or enzymatic digest assayed. Poly-L-lysine and BSA incubated with glucose showed a time-dependent increase in the amounts of fluorescence and FFI equivalence. The detection of a time-related increase in the amount of FFI or a closely related structure in enzymatically digested proteins implicates it as an in situ product on proteins which have undergone the Maillard reaction with glucose. Of physiological significance is that FFI could also be detected in human globin and serum albumin from normal individuals. Thus, proteins exposed to glucose in vitro and in vivo form FFI as an in situ glycosylation product.