Decreased transmethylation of biogenic amines after in vivo elevation of brain S-adenosyl-l-homocysteine

Epigenetic Mechanisms Involved in the Effects of Maternal Hyperhomocysteinemia on the Functional State of Placenta and Nervous System Plasticity in the Offspring

According to modern view, susceptibility to diseases, specifically to cognitive and neuropsychiatric disorders, can form during embryonic development. Adverse factors affecting mother during the pregnancy increase the risk of developing pathologies. Despite the association between elevated maternal blood homocysteine (Hcy) and fetal brain impairments, as well as cognitive deficits in the offspring, the role of brain plasticity in the development of these pathologies remains poorly studied. Here, we review the data on the negative impact of hyperhomocysteinemia (HHcy) on the neural plasticity, in particular, its possible influence on the offspring brain plasticity through epigenetic mechanisms, such as changes in intracellular methylation potential, activity of DNA methyltransferases, DNA methylation, histone modifications, and microRNA expression in brain cells. Since placenta plays a key role in the transport of nutrients and transmission of signals from mother to fetus, its dysfunction due to aberrant epigenetic regulation can affect the development of fetal CNS. The review also presents the data on the impact of maternal HHcy on the epigenetic regulation in the placenta. The data presented in the review are not only interesting from purely scientific point of view, but can help in understanding the role of HHcy and epigenetic mechanisms in the pathogenesis of diseases, such as pregnancy pathologies resulting in the delayed development of fetal brain, cognitive impairments in the offspring during childhood, and neuropsychiatric and neurodegenerative disorders later in life, as well as in the search for approaches for their prevention using neuroprotectors.

Influence of early identification and therapy on long-term outcomes in early-onset MTHFR deficiency

MTHFR deficiency is a severe inborn error of metabolism leading to impairment of the remethylation of homocysteine to methionine. Neonatal and early-onset patients mostly exhibit a life-threatening acute neurologic deterioration. Furthermore, data on early-onset patients' long-term outcomes are scarce. The aims of this study were (1) to study and describe the clinical and laboratory parameters of early-onset MTHFR-deficient patients (i.e., ≤3 months of age) and (2) to identify predictive factors for severe neurodevelopmental outcomes in a cohort with early and late onset MTHFR-deficient patients. To this end, we conducted a retrospective, multicentric, international cohort study on 72 patients with MTHFR deficiency from 32 international metabolic centres. Characteristics of the 32 patients with early-onset MTHFR deficiency were described at time of diagnosis and at the last follow-up visit. Logistic regression analysis was used to identify predictive factors of severe neurodevelopmental outcome in a broader set of patients with early and non-early-onset MTHFR deficiency. The majority of early-onset MTHFR-deficient patients (n = 32) exhibited neurologic symptoms (76%) and feeding difficulties (70%) at time of diagnosis. At the last follow-up visit (median follow-up time of 8.1 years), 76% of treated early-onset patients (n = 29) exhibited a severe neurodevelopmental outcome. Among the whole study population of 64 patients, pre-symptomatic diagnosis was independently associated with a significantly better neurodevelopmental outcome (adjusted OR 0.004, [0.002-0.232]; p = 0.003). This study provides evidence for benefits of pre-symptomatic diagnosis and appropriate therapeutic management, highlighting the need for systematic newborn screening for MTHFR deficiency and pre-symptomatic treatment that may improve outcome.

Prenatal Hyperhomocysteinemia Induces Glial Activation and Alters Neuroinflammatory Marker Expression in Infant Rat Hippocampus

Maternal hyperhomocysteinemia is one of the common complications of pregnancy that causes offspring cognitive deficits during postnatal development. In this study, we investigated the effect of prenatal hyperhomocysteinemia (PHHC) on inflammatory, glial activation, and neuronal cell death markers in the hippocampus of infant rats. Female Wistar rats received L-methionine (0.6 g/kg b.w.) by oral administration during pregnancy. On postnatal days 5 and 20, the offspring’s hippocampus was removed to perform histological and biochemical studies. After PHHC, the offspring exhibited increased brain interleukin-1β and interleukin-6 levels and glial activation, as well as reduced anti-inflammatory interleukin-10 level in the hippocampus. Additionally, the activity of acetylcholinesterase was increased in the hippocampus of the pups. Exposure to PHHC also resulted in the reduced number of neurons and disrupted neuronal ultrastructure. At the same time, no changes in the content and activity of caspase-3 were found in the hippocampus of the pups. In conclusion, our findings support the hypothesis that neuroinflammation and glial activation could be involved in altering the hippocampus cellular composition following PHHC, and these alterations could be associated with cognitive disorders later in life.

Association of methylenetetrahydrofolate reductase (MTHFR) gene C677T polymorphism with autism: evidence of genetic susceptibility

Autism (MIM 209850) is a heterogeneous neurodevelopmental disease that manifests within the first 3 years of life. Numerous articles reported that dysfunctional folate-methionine pathway enzymes may play an important role in the pathophysiology of autism. Methylenetetrahydrofolate reductase (MTHFR) is a critical enzyme of this pathway and MTHFR C677T polymorphism reported as risk factor for autism in several case control studies. However, controversial reports were also published. Hence the present meta-analysis was designed to investigate the relationship of the MTHFR C677T polymorphism with the risk of autism. Electronic databases were searched for case control studies with following search terms - 'MTHFR', 'C677T', in combination with 'Autism'. Pooled OR with its corresponding 95 % CI was calculated and used as association measure to investigate the association between MTHFR C677T polymorphism and risk of autism. Total of thirteen studies were found suitable for the inclusion in the present meta-analysis, which comprises 1978 cases and 7257 controls. Meta-analysis using all four genetic models showed significant association between C677T polymorphism and autism (ORTvs.C = 1.48; 95 % CI: 1.18-1.86; P = 0.0007; ORTT + CT vs. CC = 1.70, 95 % CI = 0.96-2.9, p = 0.05; ORTT vs. CC = 1.84, 95 % CI = 1.12-3.02, p = 0.02; ORCT vs.CC = 1.60, 95 % CI = 1.2-2.1, p = 0.003; ORTT vs.CT+CC = 1.5, 95 % CI = 1.02-2.2, p = 0.03). In total 13 studies, 9 studies were from Caucasian population and 4 studies were from Asian population. The association between C677T polymorphism and autism was significant in Caucasian (ORTvs.C = 1.43; 95 % CI = 1.1-1.87; p = 0.009) and Asian population (ORTvs.C = 1.68; 95 % CI = 1.02-2.77; p = 0.04) using allele contrast model. In conclusion, present meta-analysis strongly suggested a significant association of the MTHFR C677T polymorphism with autism.

Methylenetetrahydrofolate reductase deficiency alters levels of glutamate and γ-aminobutyric acid in brain tissue

Methylenetetrahydrofolate reductase (MTHFR) is an enzyme key regulator in folate metabolism. Deficiencies in MTHFR result in increased levels of homocysteine, which leads to reduced levels of S-adenosylmethionine (SAM). In the brain, SAM donates methyl groups to catechol-O-methyltransferase (COMT), which is involved in neurotransmitter analysis. Using the MTHFR-deficient mouse model the purpose of this study was to investigate levels of monoamine neurotransmitters and amino acid levels in brain tissue. MTHFR deficiency affected levels of both glutamate and γ-aminobutyric acid in within the cerebellum and hippocampus. Mthfr^−/− mice had reduced levels of glutamate in the amygdala and γ-aminobutyric acid in the thalamus. The excitatory mechanisms of homocysteine through activation of the N-methyl-d-aspartate receptor in brain tissue might alter levels of glutamate and γ-aminobutyric acid.

Biomarkers of a five-domain translational substrate for schizophrenia and schizoaffective psychosis

BACKGROUND

The Mental Health Biomarker Project (2010-2014) selected commercial biochemistry markers related to monoamine synthesis and metabolism and measures of visual and auditory processing performance. Within a case-control discovery design with exclusion criteria designed to produce a highly characterised sample, results from 67 independently DSM IV-R-diagnosed cases of schizophrenia and schizoaffective disorder were compared with those from 67 control participants selected from a local hospital, clinic and community catchment area. Participants underwent protocol-based diagnostic-checking, functional-rating, biological sample-collection for thirty candidate markers and sensory-processing assessment.

RESULTS

Fifteen biomarkers were identified on ROC analysis. Using these biomarkers, odds ratios, adjusted for a case-control design, indicated that schizophrenia and schizoaffective disorder were highly associated with dichotic listening disorder, delayed visual processing, low visual span, delayed auditory speed of processing, low reverse digit span as a measure of auditory working memory and elevated levels of catecholamines. Other nutritional and biochemical biomarkers were identified as elevated hydroxyl pyrroline-2-one as a marker of oxidative stress, vitamin D, B6 and folate deficits with elevation of serum B12 and free serum copper to zinc ratio. When individual biomarkers were ranked by odds ratio and correlated with clinical severity, five functional domains of visual processing, auditory processing, oxidative stress, catecholamines and nutritional-biochemical variables were formed. When the strengths of their inter-domain relationships were predicted by Lowess (non-parametric) regression, predominant bidirectional relationships were found between visual processing and catecholamine domains. At a cellular level, the nutritional-biochemical domain exerted a pervasive influence on the auditory domain as well as on all other domains.

CONCLUSIONS

The findings of this biomarker research point towards a much-required advance in Psychiatry: quantification of some theoretically-understandable, translationally-informative, treatment-relevant underpinnings of serious mental illness. This evidence reveals schizophrenia and schizoaffective disorder in a somewhat different manner, as a conglomerate of several disorders many of which are not currently being assessed-for or treated in clinical settings. Currently available remediation techniques for these underlying conditions have potential to reduce treatment-resistance, relapse-prevention, cost burden and social stigma in these conditions. If replicated and validated in prospective trials, such findings will improve progress-monitoring and treatment-response for schizophrenia and schizoaffective disorder.

Prenatal vitamins, one-carbon metabolism gene variants, and risk for autism

BACKGROUND

Causes of autism are unknown. Associations with maternal nutritional factors and their interactions with gene variants have not been reported.

METHODS

Northern California families were enrolled from 2003 to 2009 in the CHARGE (CHildhood Autism Risks from Genetics and Environment) population-based case-control study. Children aged 24-60 months were evaluated and confirmed to have autism (n = 288), autism spectrum disorder (n = 141), or typical development (n = 278) at the University of California-Davis Medical Investigation of Neurodevelopmental Disorders Institute using standardized clinical assessments. We calculated adjusted odds ratios (ORs) for associations between autism and retrospectively collected data on maternal vitamin intake before and during pregnancy. We explored interaction effects with functional genetic variants involved in one-carbon metabolism (MTHFR, COMT, MTRR, BHMT, FOLR2, CBS, and TCN2) as carried by the mother or child.

RESULTS

Mothers of children with autism were less likely than those of typically developing children to report having taken prenatal vitamins during the 3 months before pregnancy or the first month of pregnancy (OR = 0.62 [95% confidence interval = 0.42-0.93]). Significant interaction effects were observed for maternal MTHFR 677 TT, CBS rs234715 GT + TT, and child COMT 472 AA genotypes, with greater risk for autism when mothers did not report taking prenatal vitamins periconceptionally (4.5 [1.4-14.6]; 2.6 [1.2-5.4]; and 7.2 [2.3-22.4], respectively). Greater risk was also observed for children whose mothers had other one-carbon metabolism pathway gene variants and reported no prenatal vitamin intake.

CONCLUSIONS

Periconceptional use of prenatal vitamins may reduce the risk of having children with autism, especially for genetically susceptible mothers and children. Replication and mechanistic investigations are warranted.

Folate deprivation increases tau phosphorylation by homocysteine-induced calcium influx and by inhibition of phosphatase activity: Alleviation by S-adenosyl methionine

Several recent studies have indicated that increased levels of homocysteine (HC), including that resulting from deficiency in folate, increases tau phosphorylation. Some studies indicate that this is accomplished via HC-dependent activation of NMDA channels and resultant activation of calcium-dependent kinase pathways, while others suggest that the increase in tau phosphorylation is derived via HC-dependent inhibition of methylation of phosphatases and resultant inhibition of phosphatase activity. We demonstrate herein in SH-SY-5Y human neuroblastoma that both of these phenomena contribute to the increase in phospho-tau immunoreactivity following folate deprivation, and that supplementation with S-adenosyl methionine (SAM) prevents both the increase in kinase activity and the decrease in phosphatase activity. These findings demonstrate that the divergent neuropathological consequences of folate deprivation includes multiple pathways that converge upon tau phosphorylation, and further support the notion that dietary supplementation with SAM may reduce or delay neurodegeneration.

Role of S-adenosylhomocysteine hydrolase in adenosine-induced apoptosis in HepG2 cells

Adenosine has been shown to initiate apoptosis through different mechanisms: (i) activation of adenosine receptors, (ii) intracellular conversion to AMP and stimulation of AMP-activated kinase, (iii) conversion to S-adenosylhomocysteine (AdoHcy), which is an inhibitor of S-adenosylmethionine (AdoMet)-dependent methyltransferases. Since the pathways involved are still not completely understood, we further investigated the role of AdoHcy hydrolase in adenosine-induced apoptosis. In HepG2 cells, adenosine induced caspase-like activity and DNA fragmentation, a marker of apoptosis. These effects were potentiated by co-incubation with homocysteine or adenosine deaminase inhibitor, pentostatin, and were mimicked by inhibition of AdoHcy hydrolase by adenosine-2',3'-dialdehyde (Adox). Adenosine-induced effects were significantly inhibited by dipyridamole, an inhibitor of adenosine transporter, whereas inhibitors of adenosine kinase did not affect adenosine-induced changes. Various adenosine receptor agonists and AICAR, an activator of AMP-activated kinase, did not mimic the effect of adenosine. Thus, adenosine-induced apoptosis is likely due to intracellular action of AdoHcy and independent of AMP-activated kinase and adenosine receptors. Because elevated AdoHcy levels are associated with reduced mRNA methylation, we studied mRNA expression in Adox-treated cells by microarray analysis. Since several p53-target genes and other apoptosis-related genes were up-regulated by Adox, we conclude that AdoHcy is involved in adenosine-induced apoptosis by altering gene expression.

Evaluation of total plasma homocysteine in Indian newborns using heel-prick samples

OBJECTIVE

To estimate total plasma homocysteine levels in Indian newborns by modifying the existing SBD-F based High performance liquid chromotography (HPLC) method in order to enable analysis in newborn heel-prick samples and assess the prevalence of hyperhomocysteinemia in Indian newborns who are exclusively breast-fed.

METHODS

Reverse-phase HPLC with fluorescence detection for plasma homocysteine estimation and statistical analysis using student t-test.

RESULTS

SBD-F based HPLC method was modified and Bland and Altman analysis was carried out to assess agreement between original and modified methods. The correlation co-efficient was 0.994. The limits of agreement (-5.9, 6.3) were small enough to apply new method in place of the old for heel-prick sample analysis. Total plasma homocysteine analysis was carried out on heel-prick samples of 607 randomly selected newborns (331 males and 276 females). The mean plasma homocysteine estimated by this method in Indian newborns was 6.99 (95% CI: 6.48-7.49) with no appreciable gender effect (P=0.74). Elevated homocysteine levels were observed in 31 males and 21 females.

CONCLUSIONS

Modified HPLC method is validated and can be used for homocysteine analysis on newborn heel-prick samples. Using this method, the prevalence of hyperhomocysteinemia in Indian newborns is 8.6%.

Hyperhomocysteinemia and response of methionine cycle intermediates to vitamin treatment in renal patients

The role of hyperhomocysteinemia (HHcy) as a risk marker for cardiovascular diseases in renal patients is a matter of controversy. The remethylation of homocysteine (Hcy) to methionine in the kidneys is of great importance for Hcy clearance. Hcy remethylation is markedly decreased in patients on hemodialysis, but transsulfuration remains mostly unaffected. Markedly increased concentrations of methylmalonic acid (MMA), as a metabolic marker of vitamin B

Influence of an altered methylation potential on mRNA methylation and gene expression in HepG2 cells

S-adenosylhomocysteine (AdoHcy), a by-product and inhibitor of S-adenosylmethionine (AdoMet)-dependent methylation reactions, is removed by AdoHcy hydrolase. The ratio of AdoMet and AdoHcy, also termed methylation potential (MP), is a metabolic indicator for cellular methylation status. In the present study, we have investigated the influence of hypoxia and inhibition of AdoHcy hydrolase on MP in HepG2 cells. Furthermore, we studied the impact of deviations in MP on mRNA and DNA methylation and the expression of selected genes: erythropoietin, VEGF-A, AdoHcy hydrolase, cyclophilin, and HIF-1alpha. Under hypoxic conditions, the MP raised from 53.4 +/- 3.3 to 239.4 +/- 24.8, which is the result of increased AdoMet and decreased AdoHcy levels. Inhibition of AdoHcy hydrolase by adenosine-2',3'-dialdehyde leads to a 40-fold reduction of the MP under both normoxic and hypoxic conditions. Hypoxia increases erythropoietin (2.7-fold) and VEGF-A (5-fold) mRNA expression. During a reduced MP erythropoietin mRNA expression is lowered under normoxia and hypoxia by 70%, whereas VEGF-A mRNA expression is only reduced under hypoxic conditions by 60%. The mRNA expression of AdoHcy hydrolase, HIF-1alpha, and cyclophilin is insensitive to an altered MP. Furthermore, decreased MP leads to a highly significant decrease in overall mRNA methylation. Our results show that the mRNA levels of the studied genes respond differentially to changes in MP. This implies that genes with a slower transcription rate and mRNAs with a slower turnover are insensitive to short-term changes in MP.

Adenosine binding sites at S-adenosylhomocysteine hydrolase are controlled by the NAD+/NADH ratio of the enzyme

S-Adenosylhomocysteine hydrolase (AdoHcy hydrolase) catalyzes the reversible hydrolysis of S-adenosylhomocysteine (AdoHcy) to adenosine (Ado) and homocysteine. On the basis of the kinetics of Ado binding to AdoHcy hydrolase we have shown that AdoHcy hydrolase binds Ado with different affinities [Kidney Blood Press. Res. 19 (1996) 100]. Since AdoHcy hydrolase in its totally reduced form binds Ado with high affinity we determined in the present study the Ado binding characteristics of purified AdoHcy hydrolase from bovine kidney (native form) and of reconstituted forms with defined NAD(+)/NADH ratios. AdoHcy hydrolase in its native form and at a ratio of 50% NAD(+) and 50% NADH exhibits two binding sites for Ado with a K(D1) of 9.2+/-0.6 nmol/L and a K(D2) of 1.4+/-0.1 micromol/L, respectively. Binding of Ado to AdoHcy hydrolase in its NADH form and in its NAD(+) form exhibits only one binding site with high affinity 48.3+/-2.7 nmol/L for the NADH form and with a low affinity of 4.9+/-0.3 micromol/L for the NAD(+) form. To identify these two Ado binding sites, AdoHcy hydrolase was covalently modified with [2-3H]-8-azido-Ado. After irradiation of the native AdoHcy hydrolase two different photolabeled peptides were isolated and identified as Asp(307)-Val(325) and Tyr(379)-Thr(410). When the reconstituted AdoHcy hydrolase in its NADH and in its NAD(+) form was irradiated with [2-3H]-8-azido-Ado only one peptide was identified as Asn(312)-Lys(318) from the NADH form and as Asp(391)-Ala(396) from the NAD(+) form. Based on the crystallographic data, the labeled peptide Asp(391)-Ala(396) (low affinity binding site), appears to belong to the catalytic domain of AdoHcy hydrolase, whereas the labeled peptide, identified as Asn(312)-Lys(318) (high affinity binding site), is located in the NAD domain. In conclusion, our data show that AdoHcy hydrolase has two different Ado binding sites which are dependent upon the enzyme-bound NAD(+)/NADH ratios.

Moderately elevated plasma homocysteine, methylenetetrahydrofolate reductase genotype, and risk for stroke, vascular dementia, and Alzheimer disease in Northern Ireland

BACKGROUND AND PURPOSE

Elevated plasma homocysteine level has been associated with increased risk for cardiovascular and cerebrovascular disease. Variation in the levels of this amino acid has been shown to be due to nutritional status and methylenetetrahydrofolate reductase (MTHFR) genotype.

METHODS

Under a case-control design we compared fasting levels of homocysteine and MTHFR genotypes in groups of subjects consisting of stroke, vascular dementia (VaD), and Alzheimer disease patients and normal controls from Northern Ireland.

RESULTS

A significant increase in plasma homocysteine was observed in all 3 disease groups compared with controls. This remained significant after allowance for confounding factors (age, sex, hypertension, cholesterol, smoking, creatinine, and nutritional measures). MTHFR genotype was not found to influence homocysteine levels, although the T allele was found to increase risk for VaD and perhaps dementia after stroke.

CONCLUSIONS

We report that moderately high plasma levels of homocysteine are associated with stroke, VaD, and Alzheimer disease. This is not due to vascular risk factors, nutritional status, or MTHFR genotype.

Elevation in S-adenosylhomocysteine and DNA hypomethylation: potential epigenetic mechanism for homocysteine-related pathology

Chronic nutritional deficiencies in folate, choline, methionine, vitamin B-6 and/or vitamin B-12 can perturb the complex regulatory network that maintains normal one-carbon metabolism and homocysteine homeostasis. Genetic polymorphisms in these pathways can act synergistically with nutritional deficiencies to accelerate metabolic pathology associated with occlusive heart disease, birth defects and dementia. A major unanswered question is whether homocysteine is causally involved in disease pathogenesis or whether homocysteinemia is simply a passive and indirect indicator of a more complex mechanism. S-Adenosylmethionine and S-adenosylhomocysteine (SAH), as the substrate and product of methyltransferase reactions, are important metabolic indicators of cellular methylation status. Chronic elevation in homocysteine levels results in parallel increases in intracellular SAH and potent product inhibition of DNA methyltransferases. SAH-mediated DNA hypomethylation and associated alterations in gene expression and chromatin structure may provide new hypotheses for pathogenesis of diseases related to homocysteinemia.

Increase in plasma homocysteine associated with parallel increases in plasma S-adenosylhomocysteine and lymphocyte DNA hypomethylation

S-Adenosylmethionine and S-adenosylhomocysteine (SAH), as the substrate and product of essential cellular methyltransferase reactions, are important metabolic indicators of cellular methylation status. Chronic elevation of SAH, secondary to the homocysteine-mediated reversal of the SAH hydrolase reaction, reduces methylation of DNA, RNA, proteins, and phospholipids. High affinity binding of SAH to the active site of cellular methyltransferases results in product inhibition of the enzyme. Using a sensitive new high pressure liquid chromatography method with coulometric electrochemical detection, plasma SAH levels in healthy young women were found to increase linearly with mild elevation in homocysteine levels (r = 0.73; p < 0.001); however, S-adenosylmethionine levels were not affected. Plasma SAH levels were positively correlated with intracellular lymphocyte SAH levels (r = 0.81; p < 0.001) and also with lymphocyte DNA hypomethylation (r = 0.74, p < 0.001). These results suggest that chronic elevation in plasma homocysteine levels, such as those associated with nutritional deficiencies or genetic polymorphisms in the folate pathway, may have an indirect and negative effect on cellular methylation reactions through a concomitant increase in intracellular SAH levels.

Cerebrospinal fluid and plasma total homocysteine and related metabolites in children with cystathionine beta-synthase deficiency: the effect of treatment

The neurologic complications of cystathionine beta-synthase deficiency are thought to be secondary to accumulation of homocyst(e)ine in the CNS. Treatment of this disorder with betaine has been shown to improve the behavior of individuals, to reduce plasma total homocysteine, and to correct secondary abnormalities of serine. To test the hypothesis that homocyst(e)ine accumulates within the CNS and that this can be reduced by treatment with betaine, we measured total homocysteine and related metabolites in the plasma of 10 children with cystathionine beta-synthase deficiency and cerebrospinal fluid of five children before and during betaine therapy. In plasma, betaine significantly lowered total homocysteine (but not to the normal range) and had a variable effect on methionine. In the cerebrospinal fluid, total homocysteine was raised before treatment (mean 1.2 microM) and was significantly reduced by betaine (mean 0.32 microM) but not to the normal range (<0.10 microM). Cerebrospinal fluid methionine was raised before and during treatment, but betaine did not cause a significant further increase. Cerebrospinal fluid serine was significantly reduced before treatment and rose to the normal range with betaine. Cerebrospinal fluid S-adenosylmethionine was normal before treatment and rose significantly with treatment; there were no significant changes in cerebrospinal fluid 5-methyltetrahydrofolate. The demonstration of accumulation of homocysteine within the CNS lends support to the hypothesis that this may be one cause of the neurologic complications of cystathionine beta-synthase deficiency. Betaine is effective in reducing cerebrospinal fluid homocysteine, but concentrations are still significantly raised during treatment.

Hyperhomocysteinaemia; with reference to its neuroradiological aspects

Severe or even mild hyperhomocysteinaemia can cause a wide range of neurological problems. In recent years its vascular complications, including cerebral stroke, in children and young adults have gained special interest, because hyperhomocysteinaemia is treatable and recurrence of vascular incidents may be preventable. Current knowledge about biochemical mechanisms leading to hyperhomocysteinaemia, the pathogenesis of vascular pathology and neurological disfunction, and the various patterns of cerebral damage are reviewed. The significance of MRI in diagnosis, follow-up and research on hyperhomocysteinaemia is discussed.

Brain ATP:L-methionine S-adenosyltransferase (MAT), S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH): regional distribution and age-related changes

The distribution of the activity of the enzyme methionine adenosyltransferase (ATP:L-methionine S-adenosyltransferase, EC 2.5.1.6, MAT) was investigated in human postmortem brains of individuals without a known history of neuropsychiatric disorders. The brain regions were the frontal, temporal, parietal and occipital cortices, nucleus caudatus, putamen, globus pallidus, thalamus and white matter. The activities in the nucleus caudatus and putamen were approximately 25% higher than the activities in the seven other brain regions, however, not on a statistically significant level. The apparent values of MAT Km and Vmax in the parietal cortex were 11.41 +/- 3.51 microM methionine and 25.72 +/- 3.90 nmol/mg protein/h, respectively. In the frontal cortex, a significant positive correlation between age and the activity of MAT was found (r = 0.997, P < 0.01). Concerning MAT stability in the rat brain, there was a steady decrease in the activity with postmortem time in the brains kept for 0-72 h at room temperature (23 degrees C), which reached the level of significance at 24 h. The activity did not change significantly when the brains were kept for 120 h at 4 degrees C, or by freezing and thawing the tissue before analysis. In a parallel study in rats of different ages (2-22 months), a homogeneous distribution of SAM and SAH was observed in the cortex, striatum, midbrain, hypothalamus, brainstem and cerebellum. The lowest levels of SAM and the highest levels of SAH observed in the striatum gave the lowest SAM/SAH ratio. The SAH content of rat cerebral cortex was highest in the oldest group.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical pathogenesis of subacute combined degeneration of the spinal cord and brain

In humans, subacute combined degeneration of the spinal cord and brain, a primary demyelinating disease, is caused by cobalamin or methyltetrahydrofolate deficiency. Experimental studies into its pathogenesis suggest that dysfunction of the methyl-transfer pathway may be the cause. Compelling evidence for this comes from the study of inborn errors of cobalamin metabolism where deficiency of methylcobalamin, but not deoxyadenosylcobalamin, is associated with demyelination. Recent studies have focused upon inborn errors of the methyl-transfer pathway. Cerebrospinal fluid concentrations of metabolites of the methyl-transfer pathway have been measured in humans with sequential errors of the pathway and correlated with demyelination demonstrated on magnetic resonance imaging of the brain. This has provided new data suggesting that deficiency of S-adenosylmethionine is critical to the development of demyelination in cobalamin deficiency.

Association of demyelination with deficiency of cerebrospinal-fluid S-adenosylmethionine in inborn errors of methyl-transfer pathway

Long-term deficiency of cobalamin or folate causes a demyelinating disease of the brain and spinal cord. A reduced supply of methyl groups has been implicated as its cause. To examine the mechanisms of demyelination in human beings, we have studied three children with sequential inborn errors of the methyl-transfer pathway. One child had abnormal methylfolate metabolism, one abnormal methylcobalamin metabolism, and one hypermethioninaemia probably caused by methionine adenosyltransferase deficiency. Magnetic resonance imaging of the brain and measurement of cerebrospinal-fluid concentrations of 5-methyltetrahydrofolate, methionine, and S-adenosylmethionine were carried out before and after 6-12 months of appropriate treatment. Each patient had abnormal myelination before treatment; the scans suggested demyelination. The only consistent biochemical abnormality in the cerebrospinal fluid was a low concentration of S-adenosylmethionine. Treatment led to substantial clinical improvement, apparent remyelination, and increases in cerebrospinal-fluid S-adenosylmethionine concentration into the normal range. Cerebrospinal-fluid concentrations of S-adenosylmethionine and methionine were significantly lower in eight other children with errors of the methyl-transfer pathway than in an age-matched reference population (mean [95% confidence interval] standard deviation score -1.81 [0.57], p less than 0.001 for S-adenosyl methionine and -1.82 [0.19], p less than 0.001 for methionine). The concentrations of these metabolites increased to within the reference range on treatment. We have shown that demyelination is associated with cerebrospinal-fluid S-adenosylmethionine deficiency and that restoration of S-adenosylmethionine is associated with remyelination.

High-performance liquid chromatographic determination, with ultraviolet detection, of S-adenosyl-L-methionine and S-adenosyl-L-homocysteine in rat tissues and simultaneously of normetanephrine and metanephrine for phenylethanolamine-N-methyltransferase or catechol-O-methyltransferase activities

A high-performance liquid chromatographic method, with ultraviolet detection, for the determination of S-adenosyl-L-methionine and S-adenosyl-L-homocysteine, and simultaneously of normetanephrine and metanephrine, is presented. The separation was carried out by reversed-phase ion-pair isocratic chromatography. This procedure was applied to the study of the content of S-adenosyl-L-methionine and S-adenosyl-L-homocysteine in rat brain and adrenal glands and to the measurement of phenylethanolamine-N-methyltransferase or catechol-O-methyltransferase activities in rat adrenal gland.

Isocratic high performance liquid chromatographic analysis of S-adenosylmethionine and S-adenosylhomocysteine in animal tissues: the effect of exposure to nitrous oxide

A simple isocratic high performance liquid chromatographic method for the simultaneous measurement of S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) in animal tissues is described. The direct injection of perchloric acid tissue extracts and rapid resolution of both compounds in a single run reduces any sampling and analytical errors in determining the SAM/SAH ratio, a measure of methylation reactions. The method has been used to determine changes in brain SAM/SAH ratios after exposure of rats to nitrous oxide.

Effect of toluene on rat synaptosomal phospholipid methylation and membrane fluidity

This study investigated the effects of toluene (1 g/kg, 1 hr, i.p.) on rat synaptosomal phospholipid methylation (PLM), phospholipid composition, and membrane fluidity. Toluene significantly decreased basal PLM (35%) in studies using [3H]methionine [( 3H]Met) as the methyl donor; this was reflected by similar decreases in phosphatidylmonomethylethanolamine (PME) (30%). No effects were observed in either PLM reactions that used [3H]adenosylmethionine [( 3H]AdoMet) as methyl donor, or AdoMet synthetase, suggesting that toluene preferentially affects PLM reactions that derive methyl groups from [3H]Met. Also, toluene decreased synaptosomal phosphatidylethanolamine (PE) (24%), the initial substrate for PLM, and the addition of PE back to PE-depleted synaptosomes restored methyltransferase activity. Agonist-stimulated PLM using norepinephrine (NE) demonstrated that agonist-receptor coupling returned PLM to control values in synaptosomes from toluene-treated rats. NE-stimulated PLM was also blocked by propranolol (PRO), suggesting a role for toluene in receptor-mediated events. Membrane fluidity studies demonstrated that in vivo administration of toluene increased the outer synaptosomal membrane fluidity, whereas in vitro administration of toluene had no effect. Our observations support a positive relationship between increased PLM activity and increased outer, not core, membrane fluidity. These data demonstrate that specific toluene-phospholipid interactions occur in synaptosomes, resulting in altered membrane composition, function and fluidity.

Effect of chronic primidone treatment on folate-dependent one-carbon metabolism in the rat

Rats were treated chronically with primidone (100 mg/kg/12 hr, p.o.) for up to 8 weeks. The effects of this treatment on one-carbon metabolism were determined in brain and liver. Serine hydroxymethyltransferase activity increased in both brain (44%) and liver (50%). Methylenetetrahydrofolate reductase activity increased in liver (26%) with a significant correlation to the length of treatment, but in brain it was unchanged. Methyltetrahydrofolate:homocysteine methyltransferase activity increased in brain (43%) with a significant correlation to length of treatment, but in liver no effect was observed. Methionine adenosyltransferase activity in brain was significantly lower than control at only one point after 8 weeks of chronic treatment. S-Adenosylmethionine concentration in liver increased gradually (23%) during treatment. S-Adenosylhomocysteine concentrations decreased in brain (33%) and increased in liver (23%) with chronic primidone treatment. These data support the hypothesis that chronic primidone treatment leads to folate depletion through interference with folate metabolism.

Methyl group metabolism in sheep

1. Sheep have a very low intake of methyl nutrients in the post-ruminant state, due to the almost complete degradation of dietary choline by rumen microorganisms, the lack of dietary creatine and the relatively low content of methionine in microbial proteins. 2. Methylneogenesis provides a major source of labile methyl groups in post-ruminant sheep and impairment of the methylneogenesis leads to a marked reduction of the labile methyl pool. 3. S-Adenosylmethionine (AdoMet) metabolism via transmethylation is most active in sheep liver and pancreas and is regulated by the availability of methionine and intracellular ratios of AdoMet to S-adenosylhomocysteine (AdoHcy). 4. Adaptive mechanisms which arise as a consequence of the poor methyl nutrition in post-ruminant sheep are a marked reduction of labile methyl catabolism and an increase in the capacity of methylneogenesis.

Effect of pyridoxine deficiency on phospholipid methylation in rat liver microsomes

The effect of altered methionine metabolism during pyridoxine deficiency on the activity of phosphatidylethanolamine methyltransferase (EC 2.1.1.17) and the levels of phosphatidylethanolamine (PE) and phosphatidylcholine (PC) has been evaluated in rat liver microsomes. Animals fed a pyridoxine-deficient diet for 7 wk displayed a fivefold increase in the hepatic tissue level of S-adenosylhomocysteine when compared to either control or pair-fed animal counterparts. When PE methyltransferase was assayed in vitro, a significant increase in specific activity was observed using enzyme preparations from either pair-fed or pyridoxine-deficient rats. On the other hand, phospholipid levels did not conform to the measured enzyme activity. The level of PC in microsomes from either pyridoxine-deficient or pair-fed animal groups was significantly lower than that determined for the control group of rodents. However, the level of PC was noticeably lower in microsomes from pyridoxine-deficient animals than that from pair-fed animals, which received 45% of the feed intake of the control animals. In addition, the level of PE in microsomes from pair-fed and pyridoxine-deficient animals was significantly higher than that analyzed from the control animals, further confirming decreased methylation of substrate to product. It is concluded that pyridoxine deficiency may alter the methylation of phospholipid in the endoplasmic reticulum above and beyond that produced by feed restriction alone.

In vivo depletion of S-adenosyl-L-homocysteine and S-adenosyl-L-methionine in guinea pig lung after chronic S-(-)-nicotine administration

Guinea pig lung tissue is dramatically depleted of S-adenosyl-L-homocysteine (SAH) after chronic exposure (600 micrograms/h s.c. for 21 days) of animals to either R-(+)- or S-(-)-nicotine enantiomers; S-(-)-nicotine decreased lung SAH levels by 60-fold, while R-(+)-nicotine caused an 11-fold reduction of SAH, relative to control values. Lung S-adenosyl-L-methionine (SAM) levels were also reduced (15-fold and 9-fold reductions with R-(+)- and S-(-)-isomers, respectively) in nicotine-treated animals, compared to controls. These depletions are more pronounced with the S-(-)-enantiomer. Liver tissue levels of SAH and SAM are less affected, in fact, a 4-fold increase in liver SAH levels was observed after exposure of animals to R-(+)-nicotine. These results indicate that chronic exposure to nicotine may perturb important endogenous methyltransferase reactions, which could be a contributory factor in the toxicological effects produced by cigarette smoking.

Quantitative evaluation and regulation of S-adenosylmethionine-dependent transmethylation in sheep tissues

The overall rates of S-adenosylmethionine (AdoMet)-dependent transmethylation were estimated in various tissues from the initial rate of S-adenosylhomocysteine (AdoHcy) plus AdoMet accumulation after blocking hydrolysis of AdoHcy. The rates were found to differ widely among the tissues of sheep and the highest rate was in the pancreas, being 600 times higher than that in the muscle. Sheep liver possessed approximately 75% of total-body capacity for transmethylation although the transmethylation rate was approximately half that in rat liver. The minimum estimate of daily requirement of AdoMet for transmethylation for adult sheep was approximately 18 mmol, far in excess of methionine intake. Methionine loading elevated AdoMet levels only in the tissues with a high or moderate rate of transmethylation. The kinetic properties of major methyltransferases in sheep liver along with tissue distribution of AdoMet and AdoHcy suggest that transmethylation rate is subject to physiological regulation by tissue levels of AdoMet and AdoHcy.

Effect of S-adenosylhomocysteine on sulfhydryl xenobiotic transmethylases in rat liver

Rat liver cytosolic thiopurine methyltransferase and microsomal thiol methyltransferase were each found to be subject to control by the absolute molar ratio of S-adenosylmethionine to S-adenosylhomocysteine using cell-free enzyme preparations. As this ratio was lowered, inhibition of both sulfhydryl xenobiotic transmethylases occurred. On the other hand, when the ratio was decreased in vivo by the administration of D,L-homocysteine thiolactone to animals, this alteration was accompanied by an inhibition of only thiopurine methyltransferase activity. Thiol methyltransferase activity was not significantly affected after drug treatment, which would suggest that there is a compartmentalization of S-adenosylhomocysteine in the intact hepatocyte.

Evidence for a decrease in the efficiency of beta-receptor coupling to adenylate cyclase in liver membranes from sucrose-fed rats

Sucrose feeding has been shown previously to alter the plasma concentration of several factors which may regulate beta-adrenergic receptors, including corticosteroids and insulin as well as altered sympathetic nervous system (SNS) tone. For this reason we initiated a study of the effects of sucrose feeding on the beta-adrenergic receptor-adenylate cyclase system in rat liver plasma membranes. Beta-Adrenergic responsiveness was monitored by measuring isoproterenol stimulation of adenylate cyclase activity, while beta-adrenergic receptor characteristics were evaluated by analyzing [125I]iodocyanopindolol [( 125I]CYP) binding. Rats fed rat chow ad lib. supplemented by drinking water containing 10% sucrose solution exhibited a 50-75% reduction in hepatic isoproterenol-sensitive adenylate cyclase activity. This effect of sucrose was also observed in adrenalectomized (ADX) and 6-hydroxydopamine-pretreated animals, ruling out a causal role for corticosteroids or the sympathetic nervous system respectively. No effect was observed on basal, glucagon-, fluoride- or GTP-stimulated adenylate cyclase. A small but significant decrease in [125I]CYP specific binding capacity was observed in liver membranes prepared from sucrose-fed ADX rats, whereas no change in [125I]CYP binding capacity was observed in in sucrose-fed normal rats. These observations suggest that beta-receptor to adenylate cyclase coupling efficiency is decreased by the sucrose diet. The activities of two membrane-associated phospholipid methyltransferases and the content of endogenous S-adenosylmethionine in liver were reduced by sucrose feeding, implying a defect in the methylation pathway for phosphatidylcholine synthesis. The possible relationship between this latter finding and the observed decrease in beta-adrenergic receptor to adenylate cyclase coupling efficiency is discussed.

Correlations between phospholipid methylation and neuronal catecholamine transport

A change in the fluidity of biological membranes can be produced by methylation reactions which sequentially transfer methyl groups from phosphatidylethanolamine to phosphatidylcholine. Since the physical properties of membranes may affect the function of membrane-localized transport proteins, the accumulation of norepinephrine (NE) by rat cortical synaptosomes was examined in the presence of S-adenosylhomocysteine (AdoHcy) which inhibits the methylation of phospholipids. A concentration-related decrease in the uptake of [3H]NE was produced by AdoHcy with coincident decreases in the S-adenosylmethionine (AdoMet)-dependent transmethylation of phospholipids in neuronal membranes. A kinetic analysis for the effects of AdoHcy on the neuronal uptake of NE revealed a significant decrease in both the apparent Km and Vmax. Treatment of synaptosomes with adenosine, L-homocysteine thiolactone (HTL), and erythro-9(2-hydroxy-3-nonyl)adenine (EHNA) which leads to the synthesis of intracellular AdoHcy resulted in a decrease in the Vmax with no significant change in the Km. Adenosine or EHNA alone had no effect on NE uptake, but HTL alone significantly inhibited NE uptake. The data suggest that the processes of enzymatic methylation of membrane phospholipids and the transport of norepinephrine may be associated within neuronal membranes. Inhibiting phospholipid methylation reactions can reduce the efficiency of neurotransmitter removal and perhaps indirectly alter synaptic function.

Altered cell cycle distributions of cultured human lymphoblasts during cytotoxicity related to adenosine deaminase inhibition

Serial-flow cytometric analysis of DNA content of T lymphoblasts (MOLT-4) and B lymphoblasts (MGL-8) was performed to correlate the cytotoxic properties of adenosine deaminase inhibition with alterations of DNA synthesis and disruptions of the cell cycle. The addition of deoxyadenosine up to 50 mumol/L potently decreased the growth of T lymphoblasts, and these changes were enhanced with the addition of 100 mumol/L homocysteine thiolactone. These conditions caused a virtual absence of cells from S and G2M phases after 24 hours. The DNA distribution was similar in cells cultured for 24 hours in 50 mumol/L deoxyguanosine or 2.5 mumol/L hydroxyurea. These observations suggested accumulation of cells in the G1 phase. T lymphoblasts cultured with up to 50 mumol/L adenosine had a substantial decrease in growth, which was not modified by the addition of homocysteine thiolactone. Cell cycle distributions of T lymphoblasts cultured for 24 to 48 hours under these conditions showed mild decreases in the G2M population. The addition of adenosine up to 50 mumol/L decreased the growth of B lymphoblasts, and these changes were enhanced by the addition of 100 mumol/L homocysteine thiolactone. These conditions induced mild decreases in the S-phase population in B lymphoblasts. The addition of deoxyadenosine, even with homocysteine thiolactone, did not modify growth in B lymphoblasts and the cell-cycle distributions were indistinguishable from distributions of control populations after 24 and 48 hours. The observations provide independent support for a reduction of DNA synthesis associated with cytotoxicity during adenosine-deaminase inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of S-adenosyl-L-homocysteine upon sleep in p-chlorophenylalanine pretreated rats

S-Adenosyl-L-homocysteine (7 mg/kg), administered to PCPA-pretreated rats, was able to restore normal SWS and PS quantities, as well as circadian rhythms. This effect was at its maximum when SAH was injected 48 h after PCPA (400 mg/kg). These results are discussed in terms of post-synaptic synergetic control by SAH of serotonergic and noradrenergic nerve terminals via the periventricular system and caudal medulla in relation to environmental input.

Autoradiographic localization of [14C]8-S-adenosyl-L-homocysteine in rat brain

Following intracarotid injection of [14C]8-S-adenosyl-L-homocysteine ( [14C]SAH) in the rat, radioactivity was localized by autoradiographic detection in the circumventricular organs, such as the pineal gland, choroid plexuses and area postrema, which lie outside the blood-brain barrier. HPLC analysis indicated a decrease of [14C]SAH radioactivity in these structures from 1 to 45 min, after injection. These results are discussed and related to the known pharmacological properties of SAH.

Brain methylation and epileptogenesis: the case of methionine sulfoximine

A brief review of the neurochemical effects of the convulsant agent L-methionine-dl-sulfoximine (MSO) on cerebral methylation reactions is presented. Our findings point to the involvement of a number of endogenous methyl acceptor molecules, including histamine, membrane phospholipids, and membrane proteins, in the mediation of the convulsant effect. Our findings also associate the inhibition of methylations by high levels of S-adenosyl-L-homocysteine in brain with protection against MSO-induced seizures. We propose that MSO acts by eliciting the acceleration of a regulatory methylation-demethylation sequence at key molecular sites, including the benzodiazepine receptor complex, which creates an imbalance in this sequence's normal mediation of convulsant-anticonvulsant mechanisms.

A comparison of the effects of S-adenosyl-L-homocysteine on sleep in normal and pinealectomized rats

S-adenosyl-L-homocysteine (SAH) was administered to normal and pinealectomized rats previously implanted with electrodes for polygraphic recording. In normal rats, injected at 17.00 h, 7 mg/kg SAH induced a significant increase of paradoxical sleep (PS) during the night. When injected at 09.00 or 19.00 h, no effect was observed. In pinealectomized rats, SAH had no significant effect on PS amounts but seemed to resynchronize the PS rhythm. Thus, the pineal gland plays an important role in SAH effect. The results are discussed with reference to different mechanisms within the rat pineal gland and a possible implication of beta-adrenergic receptors.

A radioisotopic method for the determination of S-adenosyl-L-homocysteine in tissues in the 10(-7) M range

S-Adenosyl-L-homocysteine is able to bind to brain membranes. We used this characteristic to measure the level of S-adenosyl-L-homocysteine in rat brain tissue. The method is rapid, at the same time very sensitive (down to 10(-7) M) and specific.

In vivo elevation of mouse brain S-adenosyl-L-homocysteine after treatment with L-homocysteine

Intraperitoneal coadministration of adenosine and L-homocysteine markedly increased S-adenosyl-L-homocysteine in whole mouse brain, but further investigations showed that this elevation could also be produced following administration of L-homocysteine alone. The noted increase was maximal (+1325%) 10 min after treatment, remaining at about this level for 30-40 min before returning to control values after 180 min. Cerebral adenosine levels were decreased after treatment with L-homocysteine, adenosine, or these two substances in combination.

Biosynthesis of polyamines in mouse brain: effects of methionine sulfoximine and adenosylhomocysteine

This study examines the consequences on cerebral polyamine biosynthesis of increases and decreases in cerebral methylation. Increases were elicited by administering the convulsant agent methionine sulfoximine (MSO) and decreases by elevating in vivo the cerebral levels of the methylation inhibitor S-adenosylhomocysteine. Following the intraventricular (i.vt.) administration of one of the two possible polyamine precursors, [1,4-14C]putrescine, the specific radioactivity (sra) of the newly formed [14C]spermidine remained unchanged. Conversely, after i.vt. L-[3,4-14C]methionine, the other polyamine precursor, significantly higher sra values for [14C]spermidine and [14C]spermine were recorded in the brains of the MSO-treated animals. [14C]S-adenosylmethionine in the brain of the MSO-treated animals was also more highly labeled following [1-14C]-methionine, indicating its accelerated formation relative to controls. We also investigated the effect of the administration of adenosine + homocysteine, a treatment that results in elevated brain adenosylhomocysteine levels, on polyamine biosynthesis from [3,4-14C]-methionine. The results of these experiments show both significantly lower sra values for [14C]spermidine and [14C]spermine and significantly higher than control endogenous methionine levels, a clear sign of the existence of a retardation in the conversion of methionine to polyamines under these conditions. In conclusion, the present study demonstrates that while interference with cerebral methylation results in significant alterations of the rate of formation of the methionine moiety of spermidine and spermine, it has no effect on the entry of the putrescine moiety into the two polyamine molecules.

Possible role of increased brain methylation in methionine sulfoximine epileptogenesis: effects of administration of adenosine and homocysteine thiolactone

An intraventricular pulse of [14COOH]L-methionine to mice pretreated with the convulsant L-methionine-dl-sulfoximine (MSO) resulted in significantly higher than control specific radioactivity values of cerebral [14COOH]L-methionine (Met), [14COOH]S-adenosyl-L-methionine (AdoMet) and [14COOH]S-adenosyl-L-homocysteine (AdoHcy). MSO administration (3 hr) also decreased brain steady-state levels of Met, AdoMet, and AdoHcy. Following an intraventricular pulse of [3H-methyl]L-methionine, the levels of [3H-methyl]phosphatidylmonomethylethanolamine and of membrane associated and soluble [3H-methyl]carboxylmethylated proteins were increased over corresponding saline-treated controls. The activity of cerebral histamine N-methyltransferase was also increased after MSO treatment. The administration of a combination of adenosine and homocysteine thiolactone to MSO-pretreated animals counteracted the MSO-induced decreases in brain Met, AdoMet, and AdoHcy as well as the increase in histamine N-methyltransferase activity. In addition, administration of adenosine together with homocysteine thiolactone decreased the incidence of, and increased the latency to MSO seizures, with the most effective anticonvulsant action occurring when cerebral AdoHcy levels were at their highest.

Purine metabolism in relation to leukemia and lymphoid cell differentiation

A number of inborn errors of purine metabolism have been associated with immunodeficiency diseases. From studies to the possible mechanism(s) leading to the defects in the immune system, it appeared that the accumulation of deoxyATP and deoxyGTP and the subsequent inhibition of ribonucleotide reductase played an important role. The inhibition of methylation pathways through the accumulation of s-adenosylmethionine seems to be a second valid concept. The amount to which certain subtypes of lymphoid cells were affected by the enzyme deficiencies was strongly related to the enzymatic make-up of the cells. Lymphoid cells from different maturation stages could be affected in a specific way, depending on the different enzyme activities of these cells. Studies on human lymphoblastic leukemias showed that, related to the immunological subtype, the different leukemias could be characterized by a different enzymatic make-up. In this paper we discuss the possibilities for a specific enzyme directed chemotherapy, directed against specific subtypes of human lymphoblastic leukemias. Experimental evidence indicates that for example the adenosine deaminase inhibitor 2'deoxycoformycin can be used as a specific drug against acute lymphoblastic leukemia with the T cell phenotype.