Enzyme inhibition capacity of tannins as affected by their molecular identity and site history

Background/Question/Methods

Tannins present in plant litter hamper decomposition due to their capacity to bind enzymes causing catalytic inactivation. Based on their monomeric identity tannins can be classified as condensed tannins (CT) and hydrolysable tannins (HT), and their distribution in plants vary according to their evolutionary lineage with  most angiosperms producing a mixture of CTs and HTs  (MT),whereas gymnosperms producing CTs alone. Traditionally, CTs are thought to be more biologically reactive than HTs. However, since HTs occur later on the evolutionary time scale, contrary to the traditional knowledge based on protein precipitation assays, we hypothesized that i) mixed tannins will have similar or greater enzyme inhibition capacity as that of pure CT ii) the enzymes in soils that are acclimatized to gymnosperms will be more inhibited by MT tannins, and the inhibitory effect of CT tannins will be at par with MT tannins in soils acclimatized to gymnosperm litter. We tested the hypothesis by quantifying the inhibition of purified tannins from eight species (grouped as CTs and MTs) on almond β-glucosidase (without interference of soil). Further, two soils with prior exposure to CTs (Pinus-soil) or MTs (Acer/Quercus soil) were used to determine the percent inhibition of β-glucosidase, N-acetylglucosaminidase, and peroxidase at multiple tannin concentrations. The enzyme inactivation of various tannins was linked to their molecular structure using NMR spectroscopy.

Results/Conclusions

Generally, MTs had greater inhibition capacity of almond beta-glucosidase than CTs. Mixed tannins exhibited higher inhibition of beta-glucosidase in the Acer/Quercus soils. Similarly, mixed tannins exhibited a higher N-acetylglucosamindase inhibition capacity than condensed tannins in the Pinus soils. However, peroxidase enzyme inhibition was greatest in the Acer/Quercus soils and was independent of tannin quality. Overall tannins had a higher inhibition of hydrolase enzymes than oxidoreductases, which were tracked to the antioxidant capacity of tannins contributed by their different monomeric units and cross linkages connecting monomers. Our results suggests that the potential enzyme inhibition capacity of tannins produced by most angiosperms to be at par or higher than that of gymnosperms.