BIOCHEMICAL ASPECTS

V. BIOCHEMICAL ASPECTS

A. Glutamate

1. Absorption and Metabolism As glutamate plays a vital role in the metabolism in the body, it is synthesized in the

body to meet its demand (endogenicity) and is among those amino acids that are classified as nonessential. It is not necessary to supply these amino acids in the diet. However, the nonessential amino acids do provide twofold benefits to our well-being: they provide important sources of nitrogen and act to supplement or to conserve the essential amino acids, whose supply could otherwise be depleted.

During absorption of both free and protein-bound glutamate, a large quantity of its α-amino nitrogen appears in the portal blood as alanine. This alanine results from the transamination of glutamate to pyruvate, α-ketoglutarate being the other end product. When a large quantity of glutamate is ingested, portal plasma glutamate increases. This elevation results in an increase in the liver’s metabolism of glutamate, with the release of glucose, lactate, and glutamate into the systemic circulating plasma. Glutamate originat- ing from protein or in free form is metabolized similarly after absorption (Stegink, 1976; Stegink et al., 1973b,1975).

Glutamate is used by the organism for a wide variety of metabolic processes and plays an important role in nitrogen and energy metabolism ( Fig. 13 ).

Figure 13 Available pathways of glutamate metabolism. (From Stegink, 1984.)

The normal steps in the metabolism of glutamate include oxidative deamination, transamination, decarboxylation, and amidation. All are well established in mammals (Meister, 1974).

An extensive study on the metabolism of exogenous monododium L-glutamate was carried out in neonatal pigs using 14 C-labeled glutamate (Stegink et al., 1973a,b). Despite the enormous quantity of glutamate administered in these studies, a considerable portion did not reach the peripheral circulation but was converted to glucose and lactate by the liver before release. Similar data have been obtained in newborn primates (Boaz et al., 1974; Stegink et al., 1975).

Other studies using different routes of administration and other animal species or organ systems have given results consistent with those of the above studies. The rodent appears to metabolize glutamate somewhat differently from the pig and the primate in that metabolites such as α-keto-glutarate and acetoacetate are found in mouse plasma following administration of glutamate but not in primates (Stegink, 1976).

2. Blood–Brain Barrier Glutamate is normally present in high concentrations in the central nervous system where

it functions as an excitatory neurotransmitter. It is produced from glucose in the brain. It does not have ready access to the brain from circulation or diet. The blood–brain barrier and the very powerful glial and neuronal uptake systems for glutamate help to keep the extracellular concentration of glutamate low in the brain. The dietary consumption of glutamate has not been shown to cause neuropathology in man. (Meldrum, 1993; Ferns- trom, 1994).

Pardridge also showed that the normal plasma glutamate level is nearly four times the K m (0.04 mM) of the transport rate of glutamate to the brain; thus glutamate carriers to the brain are virtually saturated at physiological plasma levels of this amino acid. Fur- thermore, when rates of glutamate influx are compared to net glutamate release from brain, an active efflux system for glutamate appears to exist (net release is sevenfold greater than the rate of influx) (Pardridge, 1979).

The effects of glutamate administration on the glutamate concentration of regional brain areas have been studied. Blood–brain and blood–retinal barriers are impermeable to glutamate except when massive doses (greater than 2 g/kg body weight of MSG dissolved in water to yield greater than 20-fold increases over normal plasma glutamate levels) are given to infant mice or rats (Liebschutz et al., 1977).

3. Placental Barrier Stegink has examined glutamate metabolites following infusion of radioactive MSG into

pregnant monkeys. When MSG was infused at a constant rate of 1 g/h (equivalent to 0.16–0.22 g/kg body weight), maternal plasma glutamate levels increased 10- to 20-fold over baseline; fetal plasma glutamate levels, however, remained unchanged. During infu- sion, 60% of the radioactivity in the maternal circulation was in the form of glutamate. The major radioactive compounds detected in the fetal circulation were glucose and lactate, indicative of rapid metabolism of MSG by the mother. There was essentially no radioactiv- ity present as glutamate in fetal plasma. At higher rates of infusion, maternal plasma levels of glutamate rose to approximately 70 times normal; however, peak fetal plasma glutamate levels increased to less than 10 times normal (Stegink et al., 1975).

Schneider has studied the transfer of glutamate following an in vitro perfusion tech- nique of the human placenta. When the fetal perfusate was recirculated, glutamate was progressively removed from the fetal circulation so that fetal concentrations fell below maternal concentrations (Schneider et al., 1979).

Thus one can conclude that the placenta serves as an effective barrier to the transfer of glutamate by rapidly metabolizing it.

4. Glutamate Levels of Human Breast Milk Free glutamate concentrations in human milk collected from lactating women ranged from

47 to 128 µmol/dL. This is around 10 times as much free glutamate as is found in cow’s milk. To determine if glutamate concentrations could be altered by oral ingestion of MSG, six normal lactating women received 6 g of MSG in capsules with water or with Slender,

a liquid, ready-to-eat meal product. Although there was a significant increase in plasma glutamate levels, glutamate levels in milk collected over 12 h following ingestion were not affected. Glutamate levels in milk of these subjects were similar to those noted in the fasting state or in subjects receiving a lactose placebo (Stegink et al., 1972; Baker et al., 1979).

B. 5 ′-Nucleotides

1. Absorption and Metabolism Nucleotides are widely distributed throughout the body and have various functions in

metabolism. They can be synthesized from nonpurine precursor or obtained from dietary

GMP is apparently degraded enzymatically to uric acid in the rat intestine (Wilson and Wilson, 1962) or converted to allantoin (Kojima, 1974). When 25 mg/kg body weight of 5 ′-nucleotides was given orally to male rats, 70– 80% was excreted in the urine after 24 h, with most of the remainder being found in the organ-free body. When pregnant rats were given the same dosage, 0.77% 5 ′-IMP and 0.01% 5 ′-GMP were found in the fetuses after 24 h.

In humans, administration of 5 ′-nucleotides induced elevation of uric acid levels in both serum and urine, thus indicating that at least partial degradation had occurred. For example, daily administration of 2.5 g of disodium 5 ′-inosinate caused a rise in serum uric acid levels from 3.6 to 6.9 mg%, whereas urinary levels increased from 506 to 10 mg/day. As the intake of nucleotide is around 15 mg/day per person, there is little possibil- ity that this 5 ′-nucleotide becomes the cause of gout in humans (Kojima, 1974).

2. Pharmacological Effects Few studies have investigated the pharmacological effects of 5 ′-nucleotides, except for

Kojima’s report (Kojima, 1974). Administration of 500 mg/kg of 5 ′-guanylate to mice induced abnormal floor positions and also slight respiratory depression as well as depres- sion of the avoidance response. These effects were observed about 15 min after administra- tion. With disodium 5 ′-inosinate, behavioral abnormalities were apparent, but the mice became calm after 60 min. Intravenous injection of 5 ′-nucleotides (500 mg/kg) did not induce muscle relaxation or alter electroshock-induced convulsion rates. In addition, the analgesic responses of mice to a thermal stimulus were not affected by oral administration of nucleotodes.

5 ′-Nucleotides administered to rats had no diuretic effect and caused no change in gastric juice secretion. Also 10 mg/kg of disodium 5 ′-inosinate administered intravenously in- duced no significant changes in calcium, potassium, or chloride and no apparent change in sodium.

Administration of disodium 5 ′-inosinate did not affect blood pressure, heart rate, ECG, or blood flow in the hind limbs of anesthetized cats. Disodium 5 ′-guanylate did not increase blood flow.