Aldose reductase from human psoas muscle

The reaction of aldose reductase from human psoas muscle with either pyridoxal 5′-phosphate (PLP) or pyridoxal 5′-diphospho-5′-adenosine (PLP-AMP) results in a pseudo first-order 2-fold activation of the enzyme with the stoichiometric incorporation of 1 mol of either reagent per mol of enzyme. However, in addition to an increase in Vmaxthere was also an increase in Kmfor both substrate, DL-glyceraldehyde, and coenzyme, NADPH. This resulted in an overall decrease in catalytic efficiency (kcat/Km). Spectral analysis indicated that activation by both PLP and PLP-AMP was accompanied by Schiff's base formation and ε-pyridoxyllysine was identified in hydrolysates of the reduced enzyme PLP-complex. Digestion of either PLP-modified or PLP-AMP-modified aldose reductase with endoproteinase Lys-C followed by high performance liquid chromatography purification and amino acid sequencing of the pyridoxyllated peptide revealed that PLP and PLP-AMP had modified the same lysine residue. A 32-residue peptide containing the essential lysine was found to be highly homologous with a segment of the sequence of both human liver aldehyde reductase and rat lens aldose reductase. A tetrapeptide (Ile-Pro-Lys-Ser) containing the essential lysine was identical in all three enzymes. These results highlight the close structural similarity between members of the aldehyde reductase family.