3.9 Vitamin B6 metabolic pathway analysis
Vitamin B6 (VB6) usually exists in food as pyridoxal, pyridoxine, or pyridoxamine, each of which is converted into pyridoxal phosphate by the mammalian liver (Ramos, et al., 2017). VB6 is a component of co-enzymes in the human body that are involved in a variety of metabolic reactions, especially those closely related to amino acid metabolism (Ueland, McCann, Midttun, & Ulvik, 2017; Bird, 2018). According to relevant studies, vitamin B1 (thiamine), vitamin B2 (riboflavin), and vitamin B3 (nicotinic acid) are found in relatively high levels in C. fluminea , whereas the VB6 content is very low (Wang, & Liu, 2010). Combined with the transcriptome data, we analyzed the expression of genes involved in VB6 synthesis in C. fluminea, M. meretrix, andR. philippinarum. We searched and discovered 10 existing enzymes (PdxST , PNPO , PDXP , PDXK , PDIA1 ,PDIA3 , PDIA4 , PDIA5 , PDIA6 , andTXNDC10 ) of the VB6 pathway (Figure 6A). Ribulose 5-phosphate is converted to pyridoxal 5-phosphate by PdxST(K06215), and two genes (EVM0000630 and EVM0035945) in C. fluminea were responsible for the translation of PdxST.Pyridoxal 5-phosphate is converted to pyridoxal by PDXP (K13248), and EVM0004517 was responsible for the translation of PDXP . Pyridoxal can be transformed into 4-Pyridoxate or 4-Pyridoxolactone, but these pathways were blocked due to C. fluminea having lost the relevant enzymes or genes. For all genes on the VB6 pathways of C. fluminea, genes PNPO , PDXK , PDXP , PDIA1 , and PDIA4 had one copy; genes PdxST , PDIA3 ,PDIA5 , and TXNDC10 had two copies; the PDIA6 gene had three copies (Supporting Information Table S15). We suspect that the number of genes involved in the VB6 pathway is low in C. fluminea , and this may lead to the low content of VB6. Additionally, we checked the expression of homologous genes in C. fluminea, M. meretrix, and R. philippinarum. The expression levels ofPdxST , PDIA1 , PDIA3 , and PDIA4 were higher in C. fluminea than in M. meretrix and R. philippinarum (Figure 6B). The low expression of the PDXPgene may inhibit pyridoxal synthesis downstream, and it may be one of the reasons for the low expression of VB6. However, more evidence will be needed for further validation in the future.