Mechanism of butyrate binding to histone deacetylase[HDAC] correlation with chemopreventive effects and ligand selectivity

Sodium butyrate [SB] is a natural cytodifferentiating and cancer preventive agent. These actions are largely triggered by inhibition of the HDAC enzyme, thereby inducing hyperacetylation/ transcription of certain genes. Unlike the prototype HDAC inhibitor, trichostatin-A [TSA], SB offers higher selectivity on cancer cells, with a lower affinity to HDAC. The mechanisms underlying these distinct biological profiles for SB remain undefined. We currently propose for, and attempt to identify differences in the binding of SB and TSA to the HDAC binding pocket. The lowest energy conformer of SB was prepositioned on TSA binding site of HDAC. Following minimization of the best-docked S8-HDAC complex, binding profiles, conformational changes and energy calculations have been derived. TSA elicited 4 hydrogen bonds with 3 key enzyme pocket residues [His131, His132 and Tyr297; bidentated]. SB missed a hydrogen bond with Tyr297 and caused more disruption of the pocket amino acid residues His131, His132 [RMS deviation value difference of up to 0.40 A]. Besides, a looser binding to the pivotal zinc atom of HDAC was evident with SB [1 vs. 2 bonds in case of TSA]. Likewise, SB was far loosely packed in the HDAC's binding tunnel as compared to TSA. Moreover, energy computations indicated that SB had a lower binding affinity than that of TSA [-27.8 vs. -66.3 Kcal/mol]. Detailed binding differences for both ligands are described. These studies demonstrate that SB binding to HDAC confers unique catalytic, conformational and computational characteristics consistent with a lower binding affinity to HDAC and a higher selectivity on cancer cells than TSA. These newly defined binding properties of SB can further state a framework strategy for the rational development of SB-like anticancer drugs with enhanced biological and safety profiles

Toward the mechanism of valproate teratogenesis: structural basis and interactions with agents that alter folate metabolism

The hypothesis that valproate-induced neural tube defects [NTDs] may be due to interference with folate metabolic pathways was investigated by studying the possible interactions of valproate and a number of agents that modulate folate metabolism. Valproate [VPA]-induced exencephaly [an anterior NTDs] in NMRI mice was used as an animal model. A single dose of valproic acid sodium salt [300 - 500 mg/kg, s.c.] on day 8 of gestation produced a dose related increase in exencephaly rate, embryolethality, and fetal weight retardation. Supplementation with vitamin B[6]+B[12] without and with folinic acid, serine with folinic acid, and carnitine was found to reduce valproate- induced exencephaly rate. Vitamin B[6]+B[12] and methionine reduced VPA-induced fetal weight retardation and embryotoxicity, respectively. The protection was not complete and was not always dose related, and in case of carnitine, higher doses were devoid of such effects and even increased valproate-induced exencephaly. On the other hand, coadministration of valproate with low [threshold] doses of methotrexate, trimethoprim, nitrous oxide and ethanol was found to increase the incidence of exencephaly rate. Embryotoxicity was also increased as a result of such combinations except with trimethoprim. The observed effects were not due to altered valproate toxicokinetics in case of methotrexate and trimethoprim but was probably due to decreased valproate elimination by ethanol and advise against the use of these agents in valproate-treated epileptics during pregnancy. The previous results support the view that valproate-induced NTDs may be mediated via an interaction with folate metabolism. Study of the structural-activity relationships of several valproate analogues revealed a strict structural requirement for high teratogenic potency. In contrast, the anticonvulsant activity and neurotoxicity showed broader structural specificity. Furthermore, the R- and S-enantiomers of 2-n-propyl-4-pentenoic acid and 2-n-propyl-4-pentynoic acid showed different teratogenic activity [S-enantiomers were more teratogenic than R-enantiomers] in contrast to anticonvulsant potency in the absence of pharmacokinetic differences. These findings opens the possibility for development of novel antiepileptic agents with low teratogenic potency