SummaryThe immune system continuously modulates the balance between responsiveness to pathogens and tolerance to non-harmful antigens. The mechanisms that mediate tolerance are not well understood, but recent findings have implicated tryptophan catabolism through the kynurenine metabolic pathway as one of many mechanisms involved. The enzymes that break down tryptophan through this pathway are found in numerous cell types, including cells of the immune system. Some of these enzymes are induced by immune activation, including the rate limiting enzyme present in macrophages and dendritic cells, indoleamine 2,3-dioxygenase (IDO). It has recently been found that inhibition of IDO can result in the rejection of allogenic fetuses, suggesting that tryptophan breakdown is necessary for maintaining aspects of immune tolerance. Two theories have been proposed to explain how tryptophan catabolism facilitates tolerance. One theory posits that tryptophan breakdown suppresses T cell proliferation by dramatically reducing the supply of this critical amino acid. The other theory postulates that the downstream metabolites of tryptophan catabolism act to suppress certain immune cells, probably by pro-apoptotic mechanisms. Reconciling these disparate views is crucial to understanding immune-related tryptophan catabolism and the roles it plays in immune tolerance. In this review we examine the issue in detail, and offer additional insight provided by studies with antibodies to quinolinate, a tryptophan catabolite which is also necessary for nicotinamide adenine dinucleotide (NAD +) production. In addition to the immunomodulatory actions of tryptophan catabolites, we discuss the possible involvement of quinolinate as a means of replenishing NAD + in leucocytes, which is depleted by oxidative stress during an immune response.

Serotonin N-acetyltransferase, a member of the GNAT acetyltransferase superfamily, is the penultimate enzyme in the conversion of serotonin to melatonin, the circadian neurohormone. Comparison of the structures of the substrate-free enzyme and the complex with a bisubstrate analog, coenzyme A-S-acetyltryptamine, demonstrates that acetyl coenzyme A (AcCoA) binding is accompanied by a large conformational change that in turn leads to the formation of the serotonin-binding site. The structure of the complex also provides insight into how the enzyme may facilitate acetyl transfer. A water-filled channel leading from the active site to the surface provides a pathway for proton removal following amine deprotonation. Furthermore, structural and mutagenesis results indicate an important role for Tyr-168 in catalysis.

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Pineal serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, or AA-NAT) generates the large circadian rhythm in melatonin, the hormone that coordinates daily and seasonal physiology in some mammals. Complementary DNA encoding ovine AA-NAT was cloned. The abundance of AA-NAT messenger RNA (mRNA) during the day was high in the ovine pineal gland and somewhat lower in retina. AA-NAT mRNA was found unexpectedly in the pituitary gland and in some brain regions. The night-to-day ratio of ovine pineal AA-NAT mRNA is less than 2. In contrast, the ratio exceeds 150 in rats. AA-NAT represents a family within a large superfamily of acetyltransferases.

The daily rhythm in melatonin levels is controlled by cAMP through actions on the penultimate enzyme in melatonin synthesis, arylalkylamine N -acetyltransferase (AANAT; serotonin N -acetyltransferase, EC 2.3.1.87 ). Results presented here describe a regulatory/binding sequence in AANAT that encodes a cAMP-operated binding switch through which cAMP-regulated protein kinase-catalyzed phosphorylation [RRHTLPAN → RRHpTLPAN] promotes formation of a complex with 14-3-3 proteins. Formation of this AANAT/14-3-3 complex enhances melatonin production by shielding AANAT from dephosphorylation and/or proteolysis and by decreasing the K m for 5-hydroxytryptamine (serotonin). Similar switches could play a role in cAMP signal transduction in other biological systems.

Aspartoacylase (ASPA; EC 3.5.1.15) catalyzes deacetylation of N-acetylaspartate (NAA) to generate free acetate in the central nervous system (CNS). Mutations in the gene coding ASPA cause Canavan disease (CD), an autosomal recessive neurodegenerative disease that results in death before 10 years of age. The pathogenesis of CD remains unclear. Our working hypothesis is that deficiency in the supply of the NAA-derived acetate leads to inadequate lipid/myelin synthesis during development, resulting in CD. To explore the localization of ASPA in the CNS, we used double-label immunohistochemistry for ASPA and several cellspecific markers. A polyclonal antibody was generated in rabbit against mouse recombinant ASPA, which reacted with a single band (ϳ37 kD) on Western blots of rat brain homogenate. ASPA colocalized throughout the brain with CC1, a marker for oligodendrocytes, with 92-98% of CC1-positive cells also reactive with the ASPA antibody. Many cells were labeled with ASPA antibodies in white matter, including cells in the corpus callosum and cerebellar white matter. Relatively fewer cells were labeled in gray matter, including cerebral cortex. No astrocytes were labeled for ASPA. Neurons were unstained in the forebrain, although small numbers of large reticular and motor neurons were faintly to moderately stained in the brainstem and spinal cord. Many ascending and descending neuronal fibers were moderately stained for ASPA in the medulla and spinal cord. Microglial-like cells showed faint to moderate staining with the ASPA antibodies throughout the brain by the avidin/biotinperoxidase detection method, and colocalization studies with labeled lectins confirmed their identity as microglia. The predominant immunoreactivity in oligodendrocytes is consistent with the proposed role of ASPA in myelination, supporting the case for acetate supplementation as an immediate and inexpensive therapy for infants diagnosed with CD.

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Canavan's disease (CD) is a fatal, hereditary disorder of CNS development that has been linked to mutations in the gene for the enzyme aspartoacylase (ASPA) (EC 3.5.1.15). ASPA acts to hydrolyze N-acetylaspartate (NAA) into L-aspartate and acetate, but the connection between ASPA deficiency and the failure of proper CNS development is unclear. We hypothesize that one function of ASPA is to provide acetate for the increased lipid synthesis that occurs during postnatal CNS myelination. The gene encoding ASPA has been inactivated in the mouse model of CD, and here we show significant decreases in the synthesis of six classes of myelinassociated lipids, as well as reduced acetate levels, in the brains of these mice at the time of peak postnatal CNS myelination. Analysis of the lipid content of white matter from a human CD patient showed decreased cerebroside and sulfatide relative to normal white matter. These results demonstrate that myelin lipid synthesis is significantly compromised in CD and provide direct evidence that defective myelin synthesis, resulting from a deficiency of NAAderived acetate, is involved in the pathogenesis of CD.acetyl CoA ͉ leukodystrophy ͉ oligodendrocytes ͉ aspartoacylase N -acetylaspartate (NAA) attains one of the highest concentrations of any molecule in the human CNS (1), yet the functions it serves remain controversial. NAA is synthesized from L-aspartate and acetyl CoA in neuronal mitochondria by the enzyme aspartate N-acetyltransferase (Asp-NAT) (EC 2.3.1.17) (2, 3). NAA is found predominantly in neurons, (4) but the catabolic enzyme aspartoacylase (ASPA) is present primarily in oligodendrocytes in the CNS (5). The high concentration of NAA in the CNS and its characteristic peak in water-suppressed proton magnetic resonance spectroscopy (MRS) permits noninvasive determinations of NAA concentrations in the human brain. MRS determinations of NAA levels are commonly used for evaluating the integrity of neurons in a number of neurological disorders, and this method has emerged as a preferred technique for following the clinical course of several CNS pathologies (6-8). MRS studies operate on the assumptions that NAA is synthesized by and accumulated in neurons and that the steady-state NAA levels in the brain can be interpreted as indicating overall neuronal health or integrity (9, 10).Canavan's disease (CD) is a fatal, hereditary leukodystrophy that compromises normal CNS development and is caused by mutations in the gene for the enzyme ASPA (11, 12). ASPA currently is thought to function exclusively to hydrolyze NAA, a neuron-specific amino acid derivative, into L-aspartate and free acetate. However, ASPA is strongly expressed in other tissues, such as kidney, even though the only known substrate, NAA, is present predominantly in the nervous system (13). Despite the established connection between mutations in the gene for ASPA in CD and the lost capacity to deacetylate NAA, the specific connection between ASPA deficiency and the failure of proper CNS development is unclear (14). Furt...