A novel piperidine degradation mechanism in a newly isolated piperidine degrader Pseudomonas sp. strain KU43P

Detoxification and underlying mechanisms towards toxic alkaloids by Traditional Chinese Medicine processing: A comprehensive review

BACKGROUND

Alkaloids have attracted enduring interest worldwide due to their remarkable therapeutic effects, including analgesic, anti-inflammatory, and anti-tumor properties, thus offering a rich source for lead compound design and new drug discovery. However, some of these alkaloids possess intrinsic toxicity. Processing (Paozhi) is a pre-treatment step before the application of herbal medicines in traditional Chinese medicine (TCM) clinics, which has been employed for centuries to mitigate the toxicity of alkaloid-rich TCMs.

PURPOSE

To explore the toxicity phenotypes, chemical basis, mode of action, detoxification processing methods, and underlying mechanisms, we can gain crucial insights into the safe and rational use of these toxic alkaloid-rich herbs. Such insights have the great potential to offer new strategies for drug discovery and development, ultimately improving the quality of life for millions of people.

METHODS

Literatures published or early accessed until December 31, 2023, were retrieved from databases including PubMed, Web of Science, and CNKI. The following keywords, such as "toxicity", "alkaloid", "detoxification", "processing", "traditional Chinese medicine", "medicinal plant", and "plant", were used in combination or separately for screening.

RESULTS

Toxicity of alkaloids in TCM includes hepatotoxicity, nephrotoxicity, neurotoxicity, cardiotoxicity, and other forms of toxicity, primarily induced by pyrrolizidines, quinolizidines, isoquinolines, indoles, pyridines, terpenoids, and amines. Factors such as whether the toxic-alkaloid enriched part is limited or heat-sensitive, and whether toxic alkaloids are also therapeutic components, are critical for choosing appropriate detoxification processing methods. Mechanisms of alkaloid detoxification includes physical removal, chemical decomposition or transformation, as well as biological modifications.

CONCLUSION

Through this exploration, we review toxic alkaloids and the mechanisms underlying their toxicity, discuss methods to reduce toxicity, and unravel the intricate mechanisms behind detoxification. These offers insights into the quality control of herbs containing toxic alkaloids, safe and rational use of alkaloid-rich TCMs in clinics, new strategies for drug discovery and development, and ultimately helping improve the quality of life for millions of people.

Identification and characterization of a pab gene cluster responsible for the 4-aminobenzoate degradation pathway, including its involvement in the formation of a γ-glutamylated intermediate in Paraburkholderia terrae strain KU-15

Paraburkholderia terrae strain KU-15 grows on 2- and 4-nitrobenzoate and 2- and 4-aminobenzoate (ABA) as the sole nitrogen and carbon sources. The genes responsible for the potential degradation of 2- and 4-nitrobenzoate and 2-ABA have been predicted from its genome sequence. In this study, we identified the pab operon in P. terrae strain KU-15. This operon is responsible for the 4-ABA degradation pathway, which involves the formation of a γ-glutamylated intermediate. Reverse transcription-polymerase chain reaction revealed that the pab operon was induced by 4-ABA. Herein, studying the deletion of pabA and pabB1 in strain KU-15 and the examining of Escherichia coli expressing the pab operon revealed the involvement of the operon in 4-ABA degradation. The first step of the degradation pathway is the formation of a γ-glutamylated intermediate, whereby 4-ABA is converted to γ-glutamyl-4-carboxyanilide (γ-GCA). Subsequently, γ-GCA is oxidized to protocatechuate. Overexpression of various genes in E. coli and purification of recombinant proteins permitted the functional characterization of relevant pathway proteins: PabA is a γ-GCA synthetase, PabB1-B3 functions in a multicomponent dioxygenase system responsible for γ-GCA dioxygenation, and PabC is a γ-GCA hydrolase that reverses the formation of γ-GCA by PabA.

Microbial degradation of plant toxins

Plants produce a variety of secondary metabolites in response to biotic and abiotic stresses. Although they have many functions, a subclass of toxic secondary metabolites mainly serve plants as deterring agents against herbivores, insects, or pathogens. Microorganisms present in divergent ecological niches, such as soil, water, or insect and rumen gut systems have been found capable of detoxifying these metabolites. As a result of detoxification, microbes gain growth nutrients and benefit their herbivory host via detoxifying symbiosis. Here, we review current knowledge on microbial degradation of toxic alkaloids, glucosinolates, terpenes, and polyphenols with an emphasis on the genes and enzymes involved in breakdown pathways. We highlight that the insect-associated microbes might find application in biotechnology and become targets for an alternative microbial pest control strategy.

Identification and characterization of a novel class of self-sufficient cytochrome P450 hydroxylase involved in cyclohexanecarboxylate degradation in Paraburkholderia terrae strain KU-64

Cytochrome P450 monooxygenases play important roles in metabolism. Here, we report the identification and biochemical characterization of P450CHC, a novel self-sufficient cytochrome P450, from cyclohexanecarboxylate-degrading Paraburkholderia terrae KU-64. P450CHC was found to comprise a [2Fe-2S] ferredoxin domain, NAD(P)H-dependent FAD-containing reductase domain, FCD domain, and cytochrome P450 domain (in that order from the N terminus). Reverse transcription-polymerase chain reaction results indicated that the P450CHC-encoding chcA gene was inducible by cyclohexanecarboxylate. chcA overexpression in Escherichia coli and recombinant protein purification enabled functional characterization of P450CHC as a catalytically self-sufficient cytochrome P450 that hydroxylates cyclohexanecarboxylate. Kinetic analysis indicated that P450CHC largely preferred NADH (Km = 0.011 m m) over NADPH (Km = 0.21 m m). The Kd, Km, and kcat values for cyclohexanecarboxylate were 0.083 m m, 0.084 m m, and 15.9 s-1, respectively. The genetic and biochemical analyses indicated that the physiological role of P450CHC is initial hydroxylation in the cyclohexanecarboxylate degradation pathway.

Identification and characterization of a chc gene cluster responsible for the aromatization pathway of cyclohexanecarboxylate degradation in Sinomonas cyclohexanicum ATCC 51369

Cyclohexanecarboxylate (CHCA) is formed by oxidative microbial degradation of n-alkylcycloparaffins and anaerobic degradation of benzoate, and also known to be a synthetic intermediate or the starter unit of biosynthesis of cellular constituents and secondary metabolites. Although two degradation pathways have been proposed, genetic information has been limited to the β-oxidation-like pathway. In this study, we identified a gene cluster, designated chcC1XTC2B1B2RAaAbAc, that is responsible for the CHCA aromatization pathway in Sinomonas (formerly Corynebacterium) cyclohexanicum strain ATCC 51369. Reverse transcription-PCR analysis indicated that the chc gene cluster is inducible by CHCA and that it consists of two transcriptional units, chcC1XTC2B1B2R and chcAaAbAc. Overexpression of the various genes in Escherichia coli, and purification of the recombinant proteins led to the functional characterization of ChcAaAbAc as subunits of a cytochrome P450 system responsible for CHCA hydroxylation; ChcB1 and ChcB2 as trans-4-hydroxyCHCA and cis-4-hydroxyCHCA dehydrogenases, respectively; ChcC1 was identified as a 4-oxoCHCA desaturase containing a covalently bound FAD; and ChcC2 was identified as a 4-oxocyclohexenecarboxylate desaturase. The binding constant of ChcAa for CHCA was found to be 0.37 mM. Kinetic parameters established for ChcB1 indicated that it has a high catalytic efficiency towards 4-oxoCHCA compared to trans- or cis-4-hydroxyCHCA. The K_m and K_cat values of ChcC1 for 4-oxoCHCA were 0.39 mM and 44 s^-1, respectively. Taken together with previous work on the identification of a pobA gene encoding a 4-hydroxybenzoate hydroxylase, we have now localized the remaining set of genes for the final degradation of protocatechuate before entry into the tricarboxylic acid cycle.