Nicotinic acid-6-14C metabolism in man

Attenuation of the synthesis of plasminogen activator inhibitor type 1 by niacin. A potential link between lipid lowering and fibrinolysis

BACKGROUND

Plasminogen activator inhibitor type 1 (PAI-1), the primary physiological inhibitor of endogenous plasminogen activators, has been implicated as a potentiating factor in atherogenesis as well as in coronary thrombosis. We and others have observed attenuation of PAI-1 expression by gemfibrozil both in vivo and in vitro.

METHODS AND RESULTS

To determine whether other lipid-lowering agents with different mechanisms of action exert similar effects, we exposed Hep G2 cells, a highly differentiated human hepatoma cell line, to selected concentrations of niacin. Accumulation of PAI-1 protein, assayed with an ELISA, decreased in conditioned media by 72% in 48 hours in a specific, concentration-dependent fashion. Metabolic labeling experiments demonstrated a decrease in the rate of PAI-1 synthesis. Northern blot analysis demonstrated a preceding, parallel, and specific decrease in the concentration of PAI-1 mRNA. Niacin attenuated the increased PAI-1 synthesis induced by mediators released from thrombi as well. Thus, with 4.25 ng/mL transforming growth factor-beta 1, PAI-1 accumulation increased 4.5-fold in conditioned media in 48 hours. However, niacin attenuated the increase by 65%. Again, both the rate of PAI-1 synthesis and PAI-1 mRNA were reduced. The increased plasma PAI-1 activity and PAI-1 mRNA in liver induced by dexamethasone (0.8 mg IP) in vivo in rats were attenuated by 3 weeks of pretreatment with niacin.

CONCLUSIONS

These results suggest that niacin, by decreasing PAI-1 expression, may potentiate fibrinolysis, thereby decreasing the stimulation of atherogenesis by clot-associated mitogens associated with microthrombi. Furthermore, the results imply that a pathogenetic link may exist between intracellular lipid metabolism and regulation of expression of fibrinolytic system components.

Conversion of vitamin B 6 compounds to active forms in the red blood cell

In studies with pyridoxine and other B(6) compounds in blood, the active forms pyridoxal and pyridoxal phosphate were measured by differential assays using Lactobacillus casei. Red cell uptake of tritiated pyridoxine was also measured. A new metabolic pathway for conversion of pyridoxine to active forms was demonstrated in red cells. In vivo studies in normal subjects suggested that pyridoxine was taken up by red cells where it was converted to pyridoxal phosphate and then pyridoxal, followed by gradual release of a proportion of pyridoxal into plasma. In vitro incubation of pyridoxine with blood confirmed this observation. Increasing amounts of pyridoxine were taken up and converted as the amount added to blood was increased, and only very small numbers of red cells were needed to convert appreciable amounts. Conversion was markedly inhibited at temperatures lower than 37 degrees C, and stopped altogether at - 20 degrees C.Release of pyridoxal into plasma was always directly proportional to the amount of pyridoxal formed and to the volume of plasma present. That pyridoxal phosphate was not released into plasma was demonstrated in stored blood, for pyridoxine was converted mainly only as far as pyridoxal phosphate, probably due to inactivation of the phosphatase. Pyridoxal phosphate remained in the red cells. Pyridoxine was converted when incubated with washed red cells in saline or phosphate buffer suspension (0.08 M). In saline suspension, pyridoxal formed but was not released in the absence of plasma. In phosphate buffer suspension, pyridoxal phosphate was formed but was not changed to pyridoxal, probably due to inactivation of phosphatase by excess phosphate. Pyridoxamine was converted to active forms in red cells less efficiently. Pyridoxal entered red cells rapidly, equilibrating between plasma and cells within 1 min in the same ratio as pyridoxal formed inside red cells. Pyridoxal phosphate did not enter red cells in whole blood but did so readily in washed cells in saline.