Determination of niacin metabolites 1-methyl-5-carboxylamide-2-pyridone and n-1-methylnicotinamide in urine by high-performance liquid chromatography

A test suitable for detecting latent niacin deficiency was developed. It measures the 24-h urinary output of the two major metabolites of niacin, 1-methyl-5-carboxylamide-2-pyridone and N-1-methylnicotinamide. The two metabolites were isolated from urine using separate ion-exchange extractions. They and their two internal standards were quantitated simultaneously by high-performance liquid chromatography using a reversed-phase ion pairing separation. Detection was by absorbance at 254 nm.     

Metabolism of NAD and N1-methylnicotinamide in growing and growth-arrested cells

Nicotinamide is metabolized primarily into NAD and N1-methylnicotinamide in cultured cells of normal rat kidney. The metabolic pathways for the nicotinamide metabolites are independently regulated and are influenced by the growth stage of the cells. N1-Methylnicotinamide levels are 1.5--2-fold elevated in cells growth-arrested by treatment with histidinol, thymidine, or picolinic acid, or by serum starvation. This increase is due to a more rapid rate of synthesis rather than decrease in excretion. The rates of both synthesis and degradation of NAD are increased in serum-starved cells so that the NAD concentration is the same as it is in growing cells. NAD and N1- methylnicotinamide levels are not significantly increased when the intracellular nicotinamide concentration is increased 20-fold by addition of excess nicotinamide to the culture medium, demonstrating that the size of the nicotinamide pool does not limit synthesis of these compounds. In medium containing normal amounts of nicotinamide, the apparent first-order rate constant for the decay of NAD, radioactively labeled in the nicotinamide moiety, is about 4 h-1. Labeled N1-methylnicotinamide is not metabolized, but rather is excreted into the medium with a first-order rate constant of 3.9 h-1. The rate of loss of label from NAD, but not from N1-methylnicotinamide, is increased about twofold by addition of excess nicotinamide to the culture medium. This could be explained by a dilution of a labeled nicotinamide pool which is formed during NAD degradation and which is recycled into NAD but not into N1-methylnicotinamide. The results demonstrate a rapid turnover of NAD at the bond joining nicotinamide and ADP-ribose, in agreement with previous studies. In addition, the results show that nicotinamide is metabolized into N1-methylnicotinamide with what appears to be a carefully regulated synthetic mechanism. The existence of significant amounts of N1-methylnicotinamide in cultured cells raises the question of the physiological importance of this compound.     

Determination of endogenous concentrations of N1-methylnicotinamide in human plasma and urine by high-performance liquid chromatography

A high-performance liquid chromatographic assay for the determination of endogenous plasma and urine concentrations of N1-methylnicotinamide was developed. N1-Methylnicotinamide and N1-ethylnicotinamide (internal standard) are reacted with acetophenone in a strong base at 0 degree C, formic acid is added, and the reaction mixture is heated in a boiling water bath, resulting in the formation of fluorescent derivatives. These derivatives were chromatographed on a C18 reverse-phase column using a mobile phase of acetonitrile-triethylamine and 0.01 M heptanesulfonic acid adjusted to pH 3.2. Fluorescent detection was achieved using 366-nm excitation and 418-nm emission filters. Precision and accuracy were generally greater than 90%, interfering peaks did not cochromatograph, and the limit of quantification was 2 ng/ml in plasma using a 0.2-ml sample. The method was used to examine the concentrations of endogenous N1-methylnicotinamide in the plasma of 36 subjects with various pathology. The mean concentration was 18 ng/ml and the range was 6.2 to 116.7 ng/ml. The assay represents a marked improvement on previous methods and is suitable for routine clinical monitoring.     

Quantitation of the niacin metabolites 1-methylnicotinamide and l-methyl-2-pyridone-5-carboxamide in random spot urine samples, by ion-pairing reverse-phase HPLC with UV detection, and the implications for the use of spot urine samples in the assessment of niacin status

A simple ion-pairing reverse-phase HPLC method, with UV diode array detection, was developed and validated for quantitation of the urinary niacin metabolites 1-methylnicotinamide and l-methyl-2-pyridone-5-carboxamide in a single run. Urine samples were purified using a polymer-based mixed mode anion exchange reverse-phase cartridge. Analysis was performed on a reverse-phase C18 column, using a methanol gradient elution system, containing phosphate buffer pH 7.0, 1-heptanesulphonic acid as the ion-pairing agent and trimethylamine as a modifier. The assay was applied to the measurement of the niacin status of two subjects using spot urine samples. The samples were collected over 4 consecutive days and at four time points during 1 day. Status, expressed as the concentration ratios (2-PYR or 1-MN)/creatinine and 2-PYR/l-MN, varied within and between days and was least for fasting samples. This work illustrates the potential of spot urine sampling for niacin status assessment, but highlights the need for further validation prior to its use in field nutritional surveys.     

A fluorimetric method for quantitation in the picomole range of N1-methylnicotinamide and nicotinamide in serum.

A method is described for the fluorimetric determination of N¹-methylnicotinamide in deproteinized serum extract and of nicotinamide after extraction into ethyl acetate from deproteinized serum extract and subsequent conversion to N¹-methylnicotinamide. N¹-methylnicotinamide is converted to fluorescent derivatives by treatment with acetophenone in alcoholic KOH followed by addition of 99% formic acid.     

Conversion of d-erythrose to d-fructose by rat liver homogenates

d-Erythrose-4-¹⁴C has been prepared by lead tetraacetate oxidation of d-glucose-6-¹⁴C, and its metabolism by rat liver homogenates has been investigated. Only a small number of radioactive materials resulted from the action of these homogenates on the labeled tetrose. The compound which accumulated in the largest amounts has been identified as d-fructose by chromatography and by co-derivitization with the known compound. The d-fructose derived from d-erythrose-4-¹⁴C was found to contain all of its radioactivity in the 6 position. The component of the rat liver homogenate responsible for this interconversion has been shown to be heat-labile, unaffected by freezing and thawing, nonsedimentable at 100,000 g and nondialyzable. After purification of the enzyme system by centrifugation and dialysis, d-fructose 6-phosphate was required and the activity showed some stimulation by thiamine pyrophosphate. The Michaelis constant for d-erythrose was found to be 3 × 10⁻³m under the conditions of assay employed. These results are taken to indicate that the primary route of d-erythrose transformation in these homogenates proceeds by a transfer of a two-carbon fragment from transketolase or a transketolase like enzyme to the free tetrose yielding d-fructose.     

Nicotinamide methylation and its relation to NAD synthesis in rat liver tissue culture: Biochemical basis fro the physiological activities of 1-methylnicotinamide

The mode of [¹⁴C]lnicotinamide conversion to NAD and 1-methylnicotinamide and the effects of exogenous 1-methylnicotinamide on this metabolic conversion were studied using rat livers slices incubated in a chemically defined culture medium. It was shown that at the physiological nicotinamide concentrations tested (11–500 μM), 1-methylnicotinamide is preferentially produced, rather than NAD. Upon increasing nicotinamide concentration to the levels that cause cytotoxicity (1–10 mM and higher), the rate of NAD synthesis dramatically increased and reached a level 6-fold higher than that of 1-methylnicotinamide. A dose-dependent inhibition (up to 60%) of NAD synthesis was seen by the exogenous addition of 1-methylnicotinamide; the degree of inhibition is affected also by the concentrations of nicotinamide present as a precursor. A large depletion of intracellular ATP, associated with a marked accumulation of NAD, occurred in slices in response to the addition of high amounts of nicotinamide. However, loss of ATP was overcome, when nicotinamide was given together with 1-methylnicotinamide. Finally, 1-methylnicotinamide per se was proven active in regulating cell growth by comparing the cytosolic activity of 1-methylnicotinamide oxidation of cultured RLC cells with that of rat liver. Thus, the previously observed growth stimulation of hepatic cells by 1-methylnicotinamide can reasonably been explained by its ATP-sparing effect due to the inhibition of NAD synthesis, a reaction which requires ATP.