The effects of pyridoxal phosphate on rabbit liver and kidney fructose 1,6-diphosphatases

Rabbit liver and kidney fructose 1,6-diphosphatases (FDPases) rapidly become desensitized to inhibition by AMP and are partially inactivated when treated with pyridoxal 5′-phosphate. Pyridoxal is somewhat less active than pyridoxal 5′-phosphate and pyridoxamine is inactive. Inactivation and desensitization are reversed by dilution. AMP protects both enzymes against desensitization. It also protects the liver enzyme against loss of catalytic activity, but not the kidney enzyme, which is more sensitive. Pyridoxal 5′-phosphate reacts with approximately 8 ε{lunate}-amino lysine groups in liver FDPase, but the extent of desensitization is complete after incorporation of 4-5 equivalents. The amount of pyridoxal 5′-phosphate that is incorporated is increased when AMP is present. With kidney FDPase, approximately 11 equivalents of pyridoxal 5′-phosphate are incorporated, and this number is not increased by the presence of AMP. The product of the reaction with pyridoxal phosphate was characterized by spectral studies and N⁶-pyridoxyllysine was isolated from hydrolyzates of pyridoxal 5′-phosphate-enzyme complexes that had been reduced with sodium borohydride. In addition to differences in their reactions with PLP, rabbit liver and kidney FDPases differ significantly in amino acid composition. However, they appear to be similar in molecular weight and subunit structure.