Occurrence of D-aspartic acid and N-methyl-D-aspartic acid in rat neuroendocrine tissues and their role in the modulation of luteinizing hormone and growth hormone release

Using two specific and sensitive fluorometric/HPLC methods and a GC-MS method, alone and in combination with D-aspartate oxidase, we have demonstrated for the first time that N-methyl-D-aspartate (NMDA), in addition to D-aspartate (D-Asp), is endogenously present as a natural molecule in rat nervous system and endocrine glands. Both of these amino acids are mostly concentrated at nmol/g levels in the adenohypophysis, hypothalamus, brain, and testis. The adenohypophysis maximally showed the ability to accumulate D-Asp when the latter is exogenously administered. In vivo experiments, consisting of the i.p. injection of D-Asp, showed that D-Asp induced both growth hormone and luteinizing hormone (LH) release. However, in vitro experiments showed that D-Asp was able to induce LH release from adenohypophysis only when this gland was co-incubated with the hypothalamus. This is because D-Asp also induces the release of GnRH from the hypothalamus, which in turn is directly responsible for the D-Asp-induced LH secretion from the pituitary gland. Compared to D-Asp, NMDA elicits its hormone release action at concentrations approximately 100-fold lower than D-Asp. D-AP5, a specific NMDA receptor antagonist, inhibited D-Asp and NMDA hormonal activity, demonstrating that these actions are mediated by NMDA receptors. NMDA is biosynthesized from D-Asp by an S-adenosylmethionine-dependent enzyme, which we tentatively denominated as NMDA synthase.

D-aspartate disposition in neuronal and endocrine tissues: ontogeny, biosynthesis and release

High levels of D-aspartate occur in the brain and endocrine glands, such as pineal, adrenal and pituitary. In the brain, D-aspartate levels are highest in embryonic and early postnatal stages. Notably high levels occur in the early postnatal cortical plate and subventricular zone of the cerebral cortical cultures, implying a role in development. In embryonic neuronal primary culture cells, we detected high levels of endogenous D-aspartate and demonstrated biosynthesis of [14C]D-aspartate using [14C]L-aspartate as precursor. Synthesis of D-aspartate in cell cultures is inhibited by amino-oxyacetic acid, an inhibitor of pyridoxal phosphate-dependent enzymes. In the rat adrenal medulla, D-aspartate is depleted by treatment of the animals with intraperitoneal nicotine injections. In adrenal slices, D-aspartate is released by depolarization with KCl or acetylcholine, implying physiological release by activation of the cholinergic innervation of the adrenal. Our characterization of D-aspartate ontogeny, biosynthesis and depolarization-induced release implies specific physiological roles for this amino acid.

Free D-aspartate and D-alanine in normal and Alzheimer brain

In this report we present evidence for the presence of free D-aspartic acid (D-Asp) and D-alanine (D-Ala) in the white and gray matter of normal human brains and brains of individuals with Alzheimer's disease. D-Asp occurs at about the same concentration in the gray matter of both normal (18.6 nmol/g) and Alzheimer (14.8 nmol/g) brains, whereas in white matter its concentration is more than two times higher in normal than Alzheimer brains (22.4 and 10.5 nmol/g, respectively). D-Ala occurs in white matter at approximately the same concentration in both normal and Alzheimer brains (12.3 and 13.8 nmol/g, respectively), whereas in Alzheimer gray matter the D-Ala concentration is more than twice that found in normal gray matter (20.8 and 9.5 nmol/g, respectively). However, when the results are expressed as a percentage of D-amino acid/D+L, only small differences occur in all tissues examined.

Involvement of D-aspartic acid in the synthesis of testosterone in rat testes

D-Aspartic acid (D-Asp) is an endogenous amino acid which occurs in many marine and terrestrial animals. In fetal and young rats, this amino acid occurs prevalently in nervous tissue, whereas at sexual maturity it occurs in endocrine glands and above all in pituitary and testes. Here, we have studied if a relationship exists between the presence of D-Asp and the hormonal activity. The following results were obtained: 1) Both D-Asp and testosterone are synthesized in rat testes in two periods of the animal's life: before birth, about the 17th day after fertilization and, after birth, at sexual maturity. 2) Immunocytochemical studies have demonstrated that this enantiomer is localized in Leydig and Sertoli cells. 3) In vivo experiments, consisting of i.p. injection of D-Asp to adult male rats, demonstrated that this amino acid accumulates in pituitary and testis (after 5 h, the accumulation was of 12 and 4-fold over basal values, respectively); simultaneously, luteinizing hormone, testosterone and progesterone significantly increased in the blood (1.6-fold, p < 0.05; 3.0-fold, p < 0.01 and 2.9-fold, p < 0.01, respectively). 4) Finally, in vitro experiments, consisting of the incubation of D-Asp with isolated testes also demonstrated that this amino acid induces the synthesis of testosterone. These results suggest that free D-Asp is involved in the steroidogenesis.

Palmitoylethanolamide reduces pain-related behaviors and restores glutamatergic synapses homeostasis in the medial prefrontal cortex of neuropathic mice

BACKGROUND

Enhanced supraspinal glutamate levels following nerve injury are associated with pathophysiological mechanisms responsible for neuropathic pain. Chronic pain can interfere with specific brain areas involved in glutamate-dependent neuropsychological processes, such as cognition, memory, and decision-making. The medial prefrontal cortex (mPFC) is thought to play a critical role in pain-related depression and anxiety, which are frequent co-morbidities of chronic pain. Using an animal model of spared nerve injury (SNI) of the sciatic nerve, we assess bio-molecular modifications in glutamatergic synapses in the mPFC that underlie neuropathic pain-induced plastic changes at 30 days post-surgery. Moreover, we examine the effects of palmitoylethanolamide (PEA) administration on pain-related behaviours, as well as the cortical biochemical and morphological changes that occur in SNI animals.

RESULTS

At 1 month, SNI was associated with mechanical and thermal hypersensitivity, as well as depression-like behaviour, cognitive impairments, and obsessive-compulsive activities. Moreover, we observed an overall glutamate synapse modification in the mPFC, characterized by changes in synaptic density proteins and amino acid levels. Finally, with regard to the resolution of pain and depressive-like syndrome in SNI mice, PEA restored the glutamatergic synapse proteins and changes in amino acid release.

CONCLUSIONS

Given the potential role of the mPFC in pain mechanisms, our findings may provide novel insights into neuropathic pain forebrain processes and indicate PEA as a new pharmacological tool to treat neuropathic pain and the related negative affective states. Graphical Abstract Palmitoylethanolamide: a new pharmacological tool to treat neuropathic pain and the related negative affective states.

The role of D-aspartic acid and N-methyl-D-aspartic acid in the regulation of prolactin release

In this study, using an enzymatic HPLC method in combination with D-aspartate oxidase, we show that N-methyl-D-aspartate (NMDA) is present at nanomolar levels in rat nervous system and endocrine glands as a natural compound, and it is biosynthesized in vivo and in vitro. D-aspartate (D-Asp) is its natural precursor and also occurs as an endogenous compound. Among the endocrine glands, the highest quantities of D-Asp (78 +/- 12 nmol/g) and NMDA (8.4 +/- 1.2 nmol/g) occur in the adenohypophysis, whereas the hypothalamus represents the area of the nervous system where these amino acids are most abundant (55 +/- 9 and 5.6 +/- 1.1 nmol/g for D-Asp and NMDA, respectively). When D-Asp is administered to rats by ip injection, there is a significant uptake of D-Asp into the adenohypophysis and a significant increase in the concentration of NMDA in the adenohypophysis, hypothalamus and hippocampus, suggesting that D-Asp is an endogenous precursor for NMDA biosynthesis. Experiments conducted on tissue homogenates confirm that D-Asp is the precursor of the NMDA and that the enzyme catalyzing this reaction is a methyltransferase. S-adenosyl-L-methionine (SAM) is the methyl group donor. In vivo experiments consisting of ip injections of sodium D-aspartate show that this amino acid induced a significant serum PRL elevation and this effect is dose and time dependent. In vitro experiments conducted on isolated adenohypophysis or adenohypophysis coincubated with the hypothalamus, showed that the release of PRL is caused by a direct action of D-Asp on the pituitary gland and also mediated by the indirect action of NMDA on the hypothalamus. Then, the latter induces the release of a putative factor that in turn stimulates the adenohypophysis reinforcing the PRL release. In conclusion, our data suggest that D-Asp and NMDA are present endogenously in the rat and are involved in the modulation of PRL release.

Further study on the specificity of D-amino acid oxidase and D-aspartate oxidase and time course for complete oxidation of D-amino acids

1. D-Amino acid oxidase (D-AAO) oxidizes: D-Met, D-Pro, D-Phe, D-Tyr, D-Ile, D-Leu, D-Ala and D-Val. D-Ser, D-Arg, D-His, D-norleucine and D-Trp are oxidized at a low rate. D-Ornithine, cis-4-hydroxy-D-proline, D-Thr, D-Trp-methyl ester, N-acetyl-D-Ala and D-Lys are oxidized at a very low rate. 2. D-Asp, D-Glu and their derivatives, Gly and all the L-amino acids are not oxidized (or are at a rate which is undetectable). 3. Among all D-amino acids, D-Met is the most highly oxidized compound. The Km value is 1.7 mM. 4. D-Aspartate oxidase (D-Aspo) either purified from Octopus vulgaris or from beef kidney oxidizes only D-Asp, D-Glu and their following derivatives: D-Asn, D-Gln, D-Asp-dimethyl-ester and N-methyl-D-Asp. 5. However, D-Pro, D-Leu, D-Ala and D-Met, are also oxidized by this enzyme, but at a very low rate (between 0.2 and 0.6% of D-Asp). 6. All other D-amino acids, glycine and all the L-amino acids are not oxidized. 7. Under experimental conditions, 1 U of D-AAO is able to totally oxidize 0.2 micromol of the following amino acids: D-Met, D-Pro, D-Phe, D-Thy, D-Ile, D-Leu, D-Ala, D-Val, D-Ser and D-Arg. 8. Similarly, 1 U of D-AspO in 1 hr of incubation totally oxidizes 0.1 micromol of D-Asp, D-Glu, D-Asn and D-Gln.

Molecular screening of the ghrelin gene in Italian obese children: the Leu72Met variant is associated with an earlier onset of obesity

OBJECTIVE

To test whether ghrelin variants could play a role in modulating some aspects of the obese phenotype during childhood.

DESIGN

We screened the ghrelin gene in 300 Italian obese children and adolescents (mean age 10.5+/-3.2 y; range 4-19 y) and 200 controls by using the single-strand conformation polymorphism and the restriction fragment length polymoprhism analysis.

RESULTS

No mutations were detected with the exception of two previously described polymorphisms, Arg51Gln and Leu72Met. For both variations, allelic frequencies were similar between patients and controls. Interestingly, we showed that the Leu72Met polymorphism was associated with differences in the age at obesity onset. Patients with the Met72 allele became obese earlier than homozygous patients for the wild Leu72 allele. The logrank test comparing the plots of the complement of Kaplan-Meier estimates between the two groups of patients was statistically significant (P<0.0001).

CONCLUSION

It is unlikely that ghrelin variations cause the obesity due to single-gene mutations. The Leu72Met polymorphism of the ghrelin gene seems to play a role in anticipating the onset of obesity among children suggesting, therefore, that ghrelin may be involved in the pathophysiology of human adiposity.

Occurrence of sex steroid hormones and their binding proteins in Octopus vulgaris lam

The present study reports the presence of progesterone, testosterone and 17 beta-estradiol and their corresponding binding proteins in the reproductive system of Octopus vulgaris Lam (phylum Mollusca, subphylum Cephalopoda). These sex hormones occur in testis, vas deferens, seminal vesicle, prostate and Needham's sac. The hemolymph also contains a small, but significant, amount of these hormones and their carrier proteins. Among various tissues of the reproductive system, the seminal vesicle possesses the highest concentration of progesterone (4.8 ng/g tissue). The testis is the organ which contains the highest amount of testosterone (5.2 ng/g) whereas the prostate is the organ which contains the highest amount of 17 beta-estradiol (0.92 ng/g). The presence of these hormones has been ascertained by a radioimmunoassay method, an immunoenzymatic method and by a chemical (HPLC) method. Seatchard studies indicated that vas deferens and seminal vesicle contain specific sex steroid binding molecules at affinity levels comparable to those of vertebrate steroid receptors (0.5-5.0 pmol/g protein). In addition to the presence of the hormones, the delta 5,3 beta hydroxysteroid dehydrogenase, the key enzyme of steroidogenesis, also is found in testis. From a phylogenetic point of view, these findings are very interesting because they indicate a common origin of a sex hormonal system between Mollusca Cephalopoda and Vertebrates.

Secretion of D-aspartic acid by the rat testis and its role in endocrinology of the testis and spermatogenesis

The D-isomer of aspartic acid (D-Asp) has been found in rat testes. In the present study, samples of testicular venous blood plasma, rete testis fluid, interstitial extracellular fluid, luminal fluid from the seminiferous tubules, testicular parenchymal cells, epididymal spermatozoa and peripheral blood plasma were collected and analyzed for D-Asp by two methods, an enzymatic and a chromatographic HPLC method. The two methods gave very similar results for all samples. The highest concentrations of D-Asp (about 120 nmol/ml) were found in testicular venous blood plasma, with slightly lower concentrations in rete testis fluid (95 nmol/ml) and epididymal spermatozoa (80 nmol/g wet weight). Lower levels were found in testicular parenchymal cells (which would comprise mostly spermatids and spermatocytes), luminal fluid from the seminiferous tubules and interstitial extracellular fluid (26, 23 and 11 nmol/ml respectively). However, these values were all higher than those for peripheral blood plasma (6 nmol/ml). It would appear that D-Asp is being secreted by the testis mostly into the venous blood, passing thence into the rete testis fluid and being incorporated into the spermatozoa at the time or after they leave the testis. The distribution of D-Asp is thus quite different from that of testosterone, and its role and the reason for its high concentration in the male reproductive tract remain to be elucidated.

Presence of D-alanine in proteins of normal and Alzheimer human brain

This report constitutes the first demonstration of the presence of D-alanine in the proteins of the human nervous system. Proteins of the frontal lobe white and gray matter of human brains, both normal and Alzheimer subjects, contain D-alanine at concentrations between 0.50 and 1.28 mumol/g of wet tissue, 50-70-times lower than the concentration of L-alanine. Both white and gray matter of Alzheimer brains contain D-alanine 1.4-times higher than the respective regions of normal brains. The gray matter proteins of Alzheimer brains show a highly significant 8% decrease in total alanine content, when compared with normal brain gray matter proteins. Since Alzheimer's disease is exhibited by deterioration of the gray matter, the occurrence of elevated D-alanine levels in the gray matter of Alzheimer brains is a significant discovery and raises the question whether this enantiomer causes the degeneration of the gray matter proteins in Alzheimer's disease, or whether it is an effect of the disease.

Regional decreases of free D-aspartate levels in Alzheimer's disease

N-methyl-D-aspartate (NMDA) receptors have been shown to be involved in learning and memory processes. In Alzheimer's disease, there is a reduction of NMDA receptors. Since D-aspartate is an endogenous agonist for the NMDA receptor, we hypothesised that if there are reduced levels of this amino acid in the Alzheimer's brain, this could raise the reduction of NMDA receptor signal transduction system and contribute to the marked memory deficits seen in these patients. Therefore, using a chromatographic HPLC method, the regional distribution of free D-aspartate levels in post-mortem human brain samples from patients with Alzheimer's disease (AD) (n = 5) and age-matched controls (n = 5) were determined. We found that the levels of D-aspartate are significantly lower in Alzheimer's patients compared to controls (range: from -35 to -47%; P < 0.01). However, no differences were found in the cerebellum, a region spared from the neuropathological changes of AD. These data suggest that decreased levels of D-aspartate could contribute to a lower NMDA receptor function and consequently contribute to the memory deficits seen in AD.

D-aspartic acid is implicated in the control of testosterone production by the vertebrate gonad. Studies on the female green frog, Rana esculenta

In the present study we report the occurrence of D-aspartic acid (D-Asp) in the ovary of the green frog Rana esculenta and its putative involvement in testosterone production by the gonad. In the ovary, D-Asp concentrations undergo significant variations during the main phases of the sexual cycle. In spawning females (March), its concentration was low (2.5 +/- 1.1 nmol/g ovary) and during the post-reproductive period (June) it increased and reached its peak level (58.0 +/- 10.1 nmol/g) in October. In that month, vitellogenesis occurs in a new set of ovarian follicles and continues until the next spring. The concentrations of D-Asp in the ovary and of testosterone in the ovary and in the plasma were inversely correlated during the reproductive cycle: when endogenous D-Asp was low (March), testosterone was high (36.9 +/- 4.8 ng/g ovary; 23.1 +/- 2.76 ng/ml plasma) and, in contrast, when the D-Asp concentration was high (October), the testosterone concentration was low (0.86 +/- 0.21 ng/g ovary and 5.0 +/- 1.3 ng/ml plasma). In vivo experiments, consisting of injection of D-Asp (2.0 mumol/g body weight) into the dorsal lymphatic sac of adult female frogs, demonstrated that this amino acid accumulates significantly in the ovary. After 3 h, moreover, it caused a decrease in testosterone level in the plasma of about 80%. This inhibition was reversible: within 18 h after the amino acid injection, as the D-Asp concentration in the ovary decreased, the testosterone titre was restored in both ovary and plasma. In vitro experiments, conducted in isolated ovarian follicles, confirmed this phenomenon and identified these gonadal components as the putative D-Asp targets. Other amino acids (L-Asp, D-Glu, L-Glu, D-Ala and L-Ala) used instead of D-Asp were ineffective. These findings indicate that D-Asp is involved in the control of androgen secretion by the ovary in this amphibian species, revealing a more complex system for control of this androgen synthesis than was previously believed to exist.

Free D-amino acids in human cerebrospinal fluid of Alzheimer disease, multiple sclerosis, and healthy control subjects

This is the first report of the presence of free D-amino acids in lumbar and ventricular human cerebrospinal fluid (CSF) of individuals with Alzheimer disease (AD) compared with CSF of normal control subjects and with individuals affected by multiple sclerosis, as an unrelated neurologic disorder. Free D-amino acids are present at significantly higher levels in AD CSF than normal CSF, whereas in the CSF of patients affected by multiple sclerosis, D-amino acids occurs at the same level as in the normal controls. The total D-amino acid content in ventricular CSF was 1.48 times higher in the AD than controls (26.4 vs 17.9 nmol/mL, p = 0.025). The total D-amino acid content was 1.43 times higher in AD lumbar CSF than controls (1.89 vs 1.32 nmol/mL, p = 0.001). D-Aspartate in particular was 2.74 times higher in AD ventricular CSF compared to normal ventricular CSF (3.34 vs 1.22 nmol/mL, p = 0.029). In lumbar CSF, D-aspartate was 1.5 times higher in AD than controls (0.054 vs 0.036 nmol/mL, p = 0.041). Previously we reported that D-amino acids are elevated in AD brain proteins associated with neurofibrillary tangles compared to normal brain proteins (D'Aniello et al., 1992c; Fisher et al., 1992a,b). Thus, the D-amino acids present in CSF may originate from degradation of brain proteins.

Low glycemic index diet in children and young adults with refractory epilepsy: first Italian experience

This is the report on the first Italian experience with the low glycemic index diet (LGIT) in a group of children, adolescents and young adults with refractory epileptic encephalopathies. A retrospective chart review was performed on patients initiating the LGIT in an outpatient setting from 2005 to 2010. Demographic and clinical information including seizure type, baseline seizure frequency, medications, blood chemistry, side effects, and anthropometrics were collected. Patients were educated and followed by a dietician to restrict foods with high glycemic index and to limit total daily carbohydrates to 40-60g. Change in seizure frequency was assessed at each 3-month follow-up intervals in the first year and then at each 6-month intervals. Fifteen consecutive patients (13 males and 2 females, aged between 11.3 years and 22 years), almost all affected by generalized cryptogenic or symptomatic refractory epilepsy, were enrolled in the study. After a mean follow-up period of 14.5±6.5 months (median 12.0; range 1-60 months), 6 patients (40%) had a 75-90% seizure reduction, while seizures decreased by 50% in other 2 (13.3%) and were unchanged in 7 (46.7%). The diet was discontinued in 4 patients within the first 5 months. No adverse events occurred during the diet. In conclusion, this initial experience confirms that some refractory patients may improve on the LGIT, even as first dietary option.

Free D-serine concentration in normal and Alzheimer human brain

We have analyzed both free L- and D-serine in frontal cortex of normal and Alzheimer human brain by high-performance liquid chromatography (HPLC). There was no significant difference between the two brains. In normal brain, L- and D-serine concentrations were 666 +/- 222 and 66 +/- 41 nmol/g of wet tissue, respectively, and the ratio of D-isomer to L-isomer (D/L) was 0.099 +/- 0.031. In Alzheimer brain, the concentrations were 750 +/- 150 and 66 +/- 40 nmol/g, respectively, and the D/L ratio was 0.086 +/- 0.040. Thus, it was shown that the free D-serine concentration in the Alzheimer brain was comparable to that in the normal brain.

Mammalian and chicken I forms of gonadotropin-releasing hormone in the gonads of a protochordate, Ciona intestinalis

Two forms of gonadotropin-releasing hormone (GnRH) were isolated from the gonads of the tunicate, Ciona intestinalis. The primary structure of the purified peptides was determined by MS and chemical sequence analysis. Both GnRH forms have blocked NH(2) and COOH termini, and their primary structures are identical to mammalian (mGnRH) and chicken I (cGnRH-I) forms reported previously in vertebrates. A total of 1.2 mg of purified cGnRH-I and 0.98 mg of mGnRH was obtained from 100 g of Ciona gonads. The physiological effects of native GnRHs included the induction of synthesis and secretion of sex steroids from ciona gonads and the secretion of luteinizing hormone from rat pituitary. These results suggest that the primary structure and functional roles of mGnRH and cGnRH-I have been highly conserved throughout evolution of chordates.

Enhancement of aromatase activity by D-aspartic acid in the ovary of the lizard Podarcis s. sicula

The present study investigated the role of D-aspartic acid (D-Asp) in ovarian steroidogenesis and its effect on aromatase activity in the lizard, Podarcis s. sicula. It was determined that D-Asp concentrations vary significantly during phases of the reproductive cycle: they vary inversely with testosterone concentrations and directly with oestradiol concentrations in the ovary and plasma. Experimental treatment showed that administration of D-Asp induces a decrease in testosterone and an increase in oestradiol, and that treatment with other amino acids (L-Asp, D-Glu and D-Ala) instead of D-Asp has no effects. Experiments in vitro confirmed these results. Furthermore, these experiments showed an increase in aromatase activity, as the addition of D-Asp either to fresh ovarian tissue homogenate or to acetonic powder of ovarian follicles induced a significant increase in the conversion of testosterone to oestradiol. Aromatase activity is four times greater in the presence of D-Asp than in its absence. However, almost equivalent values of the two K(m) values (both approximately 25 nmol l(-1)) indicate that aromatase has the same catalytic properties in both cases.

Quantification of D-aspartate in normal and Alzheimer brains

Using a new procedure to hydrolyze proteins without provoking racemization of the amino acids and using enzymatic methods to determine D- and L-aspartate (Asp), we have quantified the content of protein-bound D-aspartate (both D-aspartic acid and D-asparagine) of human brain white and gray matter proteins from normal and Alzheimer subjects. The D-enantiomer is present in brain proteins at mean concentrations between 0.48 and 0.90 mumol/g of wet tissue, corresponding to concentrations 34-82 times lower than that of L-aspartate. The highest levels of D-aspartate were found in Alzheimer gray matter (0.60-0.90, mean 0.69 mumol/g of wet tissue). When expressed as the percentage of total (i.e. D- plus L-) aspartate, %D = [D/(D + L)] x 100, the Alzheimer brains show a significantly higher content of D-aspartate in both gray matter (2.08%) and white matter (1.80%) than in the corresponding tissues of normal brains (1.65% in gray, 1.58% in white).

Properties of the flavoenzyme D-aspartate oxidase from Octopus vulgaris

The properties of D-aspartate oxidase from Octopus vulgaris (EC 1.4.3.1) have been investigated. The protein is a monomer of M(r) 37,000 containing one mol flavin/mol protein. The enzyme as isolated exists at least in two forms, one containing FAD and the other, which is catalytically inactive, probably containing 6-OH-FAD, as inferred from the absorption spectrum of the enzyme. An additional form of the enzyme, as far as the nature of the coenzyme is concerned, has been detected in the purified enzyme and shown to derive from the form originally containing FAD. The modulation of the coenzyme reactivity exerted by Octopus D-aspartate oxidase, as studied by spectrophotometric techniques, conforms to the one expected for an enzyme belonging to the oxidase class of flavoproteins. Structural investigations show similarities in both the amino-acid composition and the N-terminal amino-acid sequence to bovine D-aspartate oxidase and porcine D-amino-acid oxidase. In summary, the general properties of the enzyme from Octopus vulgaris closely resemble those of the enzyme from beef kidney. Moreover, kinetic analyses suggest that two active-site residues with pKa of 7.1 and 9.1 are critical for catalysis, and that the ionization of such residues has different effects on the catalytic activity depending whether mono- or dicarboxylic D-amino acids are used as substrate.

A fast and sensitive method for measuring picomole levels of total free amino acids in very small amounts of biological tissues

In the present study we describe a simple and fast method to measure the concentration of total free amino acids in very small amounts of biological tissues. The procedure described here is based on the reaction of free amino acids with o-phthaldialdehyde (OPA) in the presence of a reducing agent, beta-mercaptoethanol (MET), to give a complex which can be measured by fluorescence. It is a very rapid process and has the same reliability as the conventional ninhydrin method of Moore and Stein but is about 500 times more sensitive. The sensitivity of the new protocol is such to permit the determination with high reliability of very small amounts of free amino acids at picomole levels, either in a standard amino acid mixture or in biological tissues, without chromatographic separation of the amino acids. It is particularly useful when the amount of the sample is very low, e.g. on a single pituitary or pineal gland of small animals or on single cells, such as oocytes or eggs, as well as single ganglions or axons of marine invertebrates.

Occurrence of free D-aspartic acid in the circumsoesophageal ganglia of Aplysia fasciata

This study reports the presence of high concentrations of free D-aspartic acid (D-Asp) in the circumoesophageal ganglia of the opisthobranch mollusc Aplysia fasciata. D-Asp was discovered using specific methods that employ Octopus D-aspartate oxidase and hog kidney D-amino acid oxidase to measure D-Asp levels. The concentration of D-Asp was 0.281 mumol/g wet tissue weight, which was 8.3% of the total free aspartic acid (D and L forms) present. No other free D-amino acids that were oxidised by D-amino acid oxidase were detected. To our knowledge the only molluscs that have previously been shown to have D-Asp within their nervous tissue are the cephalopods Octopus vulgaris, Loligo vulgaris and Sepia officinalis. In these, as in A. fasciata, no other D-amino acids were detected within the nervous tissue. The fact that free D-Asp occurs specifically in the nervous tissue of the above molluscs suggests that it may have a neurological function that is yet to be described.