in humans, the pyridoxine 5′-phosphate oxidase enzyme exhibits a low catalytic rate constant of 0.2 s, with low "k" values for both pyridoxine 5′-phosphate and pyridoxamine 5′-phosphate. the enzyme also has a low turnover rate, meaning that it is relatively slow converting substrate to product. pyridoxal 5′-phosphate is an effective product inhibitor. since pyridoxal 5′-phosphate, the active form of vitamin b, is the product of the metabolic pathway, if it exists in excess, then the pathway need not proceed to keep making product. however, if it exists in low concentrations, then that is a signal for the pathway to synthesize more. this is an example of feedback inhibition.