Compartmentalization of pyridoxal-5′-phosphate during the acute phase of myocardial infarction

Summary Vitamin B6 nutritional status is assessed by measuring the plasma concentration of one of its vitamers, pyridoxal-phosphate (PLP). Several conditions, e.g., myocardial infarction (MI), can disturb the dynamic equilibrium between the different vitamers resulting in transiently low plasma PLP levels. An important question is whether these low plasma PLP levels observed during MI represent a transient state of deficiency and what the possible clinical consequences of such a fall in plasma PLP could be. Since the main metabolic function of PLP is to act as an intracellular coenzyme, it was decided to monitor the changes not only of PLP but also of PL (transport form of vitamin B6) in both the plasma and red blood cells (RBCs) in patients with myocardial infarctions (MI): 16 patients with proven MI were investigated measuring the aforementioned parameters at regular intervals. It was found that the approximately 40% fall in plasma PLP levels was accompanied by an equivalent increase in RBC PLP levels. Subsequently plasma PLP concentrations returned to normal but RBC PLP values were maintained at the newly elevated steady state (without any vitamin supplementation). Since membranes are impermeable to PLP, the only way in which PLP could have been redistributed to the intracellular compartment was through hydrolysis to PL and rephosphorylation once inside the RBCs. This compartmentalization could be an important adaptive response since it has been shown that PLP reduces O₂ affinity of deoxygenated hemoglobin, thereby facilitating O₂ delivery to the tissues. In addition it would also enhance the proposed storage function of RBCs for PLP and PL which is said to be an important source for muscle vitamin B6 which again is essential for energy production by the muscle cells during ischemia.Abbreviations AST Aspartate Aminotransferase (EC 2.6.1.1) - CK Creatine Kinase (EC 2.7.3.2) - CV coefficient of variation - HPLC high performance liquid chromatography - LD lactate dehydrogenase (EC 1.1.1.27) - MI myocardial infarction - MRC Medical Research Council - NAD⁺ oxidized nicotinamide adenine dinucleotide - NADH reduced nicotinamide adenine dinucleotide - PL pyridoxal - PLP pyridoxal-5-phosphate - PLSC pyridoxal semicarbazone - PLPSC pyridoxal-5-phosphate semicarbazone - PM pyridoxamine - PMP(PNP)-oxidase pyridoxamine-phosphate- (pyridoxine-phosphate) — oxidase - PN pyridoxine - RBCs red blood cells - SEM standard error of the mean