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Hydrolysis of nicotinamide-adenine dinucleotide by purified renal brush-border membranes. Mechanism of NAD+ inhibition of brush-border membrane phosphate-transport activity.

ABSTRACT: Purified rat renal brush-border membrane vesicles possess a heat-labile enzyme activity which hydrolyses NAD+. A reciprocal relationship exists between the disappearance of NAD+ and the appearance of adenosine; 2 mol of Pi are liberated from each mol of NAD+ incubated with brush-border membrane vesicles. Freezing and thawing brush-border membrane vesicles does not enhance the initial rate of NAD+ hydrolysis. Preincubation of brush-border membrane vesicles with NAD+ results in inhibition of Na+-dependent Pi-transport activity, whereas Na+-dependent glucose transport is not affected. EDTA, which prevents the release of Pi from NAD+ and which itself has no direct effect on brush-border membrane Pi transport, reverses the NAD+ inhibition of Na+-dependent Pi transport. These results suggest that it is the Pi liberated from NAD+ and not NAD+ itself that inhibits Na+-dependent Pi transport.

SUBMITTER: Tenenhouse HS

PROVIDER: S-EPMC1158401 | biostudies-other | 1982 Jun

REPOSITORIES: biostudies-other

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Project description:One of the biochemical abnormalities found in diabetic tissues is a decrease in the cytosolic oxidized to reduced forms of the nicotinamide adenine dinucleotide ratio (NAD+ /NADH also known as pseudohypoxia) caused by oxidation of excessive substrates (glucose through the polyol pathway, free fatty acids and lactate). Subsequently, a decline in NAD+ levels as a result of the activation of poly adenine nucleotide diphosphate-ribose polymerase (mainly in type 1 diabetes) or the inhibition of adenine nucleotide monophosphate-activated protein kinase (in type 2 diabetes). Thus, replenishment of NAD+ levels by nicotinamide-related compounds could be beneficial. However, these compounds also increase nicotinamide catabolites that cause oxidative stress. This is particularly troublesome for patients with diabetes, because they have impaired nicotinamide salvage pathway reactions at the level of nicotinamide phosphoribosyl transferase and phosphoribosyl pyrophosphate, which occurs by the following mechanisms. First, phosphoribosyl pyrophosphate synthesis from pentose phosphate pathway is compromised by a decrease in plasma thiamine and transketolase activity. Second, nicotinamide phosphoribosyl transferase expression is decreased because of reduced adenosine monophosphate-activated protein kinase activity, which occurs in type 2 diabetes. The adenosine monophosphate-activated protein kinase inhibition is caused by an activation of protein kinase C and D1 as a result of enhanced diacylglycerol synthesis caused by pseudohypoxia and increased fatty acids levels. In this regard, nicotinamide-related compounds should be given with caution to treat diabetes. To minimize the risk and maximize the benefit, nicotinamide-related compounds should be taken with insulin sensitizers (for type 2 diabetes), polyphenols, benfotiamine, acetyl-L-carnitine and aldose reductase inhibitors. The efficacy of these regimens can be monitored by measuring serum NAD+ and urinary nicotinamide catabolites.

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Hydrolysis of nicotinamide-adenine dinucleotide by purified renal brush-border membranes. Mechanism of NAD+ inhibition of brush-border membrane phosphate-transport activity.

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