Oxidation of Procyanidins with Various Degrees of Condensation: Influence on the Color-Deepening Phenomenon

Metabolomics Reveals Antioxidant Metabolites in Colored Rice Grains

Colored rice is richer in nutrients and contains more nutrients and bioactive substances than ordinary white rice. Moderate consumption of black (purple) rice has a variety of physiological effects, such as antioxidant effects, blood lipid regulation, and blood sugar control. Therefore, we utilized nontargeted metabolomics, quantitative assays for flavonoid and phenolic compounds, and physiological and biochemical data to explore the correlations between metabolites and the development of antioxidant characteristics in pigmented rice seeds. The findings indicated that, among Yangjinnuo 818 (YJN818), Hongnuo (HN), Yangchannuo 1 hao (YCN1H), and Yangzi 6 hao (YZ6H), YZ6H exhibited the highest PAL activity, which was 2.13, 3.08, and 3.25 times greater than those of YJN818, HN, and YCN1H, respectively. YZ6H likewise exhibited the highest flavonoid content, which was 3.8, 7.06, and 35.54 times greater than those of YJN818, HN, and YCN1H, respectively. YZ6H also had the highest total antioxidant capacity, which was 2.42, 3.76, and 3.77 times greater than those of YJN818, HN, and YCN1H, respectively. Thus, purple rice grains have stronger antioxidant properties than other colored rice grains. Receiver operating characteristic (ROC) curve analysis revealed that trans-3,3',4',5,5',7-hexahydroxyflavanone, phorizin, and trilobatin in the YZ6H, HN, and YCN1H comparison groups all had area under the curve (AUC) values of 1. Phlorizin, trans-3,3',4',5,5',7-hexahydroxyflavanone, and trilobatin were recognized as indices of antioxidant capability in colored rice in this research. This research adds to the understanding of antioxidant compounds in pigmented rice, which can increase the nutritional value of rice and promote the overall well-being of individuals. This type of information is of immense importance in maintaining a balanced and healthy diet.

The physiological and biochemical responses to dark pericarp disease induced by excess manganese in litchi

Dark pericarp disease (DPD), a physiological disorder induced by excess Manganese (Mn) in litchi, severely impacts the appearance and its economic value. To elucidate the underlying mechanisms of DPD, this study investigated the variations of phenolic compound, antioxidant defense system, subcellular structure, and transcriptome profiles in both normal fruit and dark pericarp fruit (DPF) at three developmental stages (green, turning, and maturity) of 'Guiwei' litchi. The results reveal that excess Mn in DPF pericarp resulted in a significant increase in reactive oxygen species, especially H2O2, and subsequent alterations in antioxidant enzyme activities. Notably, SOD (EC 1.15.1.1) activity at the green stage, along with POD (EC 1.11.1.7) and APX (EC 1.11.1.11) activities at the turning and the maturity stages, and GST (EC 2.5.1.18) activity during fruit development, were markedly higher in DPF. Cell injury was observed in pericarp, facilitating the formation of dark materials in DPF. Transcriptome profiling further reveals that genes involved in flavonoid and anthocyanin synthesis were up-regulated during the green stage but down-regulated during the turning and maturity stages. In contrast, PAL (EC 4.3.1.24), C4H (EC 1.14.14.91), 4CL (EC 6.2.1.12), CAD (EC 1.1.1.195), and particularly POD, were up-regulated, leading to reduced flavonoid and anthocyanin accumulation and increased lignin content in DPF pericarp. The above suggests that the antioxidant system and phenolic metabolism jointly resisted the oxidative stress induced by Mn stress. We speculate that phenols, terpenes, or their complexes might be the substrates of the dark substances in DPF pericarp, but more investigations are needed to identify them.

Metabolite and Transcriptome Profiles of Proanthocyanidin Biosynthesis in the Development of Litchi Fruit

The fruit of Litchi chinensis contains high levels of proanthocyanidins (PAs) in the pericarp. These substances can serve as substrates of laccase-mediated rapid pericarp browning after the fruit is harvested. In this study, we found that the major PAs in litchi pericarp were (-)-epicatechin (EC) and several procyanidins (PCs), primarily PC A2, B2, and B1, and the EC and the PC content decreased with the development of the fruit. RNA-seq analysis showed that 43 early and late structure genes related to flavonoid/PA biosynthesis were expressed in the pericarp, including five ANTHOCYANIDIN REDUCTASE (ANR), two LEUCOANTHOCYANIDIN REDUCTASE (LAR), and two ANTHOCYANIDIN SYNTHASE (ANS) genes functioning in the PA biosynthesis branch of the flavonoid pathway. Among these nine PA biosynthesis-related genes, ANR1a, LAR1/2, and ANS1 were highly positively correlated with changes in the EC/PC content, suggesting that they are the key PA biosynthesis-related genes. Several transcription factor (TF) genes, including MYB, bHLH, WRKY, and AP2 family members, were found to be highly correlated with ANR1a, LAR1/2, and ANS1, and their relevant binding elements were detected in the promoters of these target genes, strongly suggesting that these TF genes may play regulatory roles in PA biosynthesis. In summary, this study identified the candidate key structure and regulatory genes in PA biosynthesis in litchi pericarp, which will assist in understanding the accumulation of high levels of browning-related PA substances in the pericarp.

Bioactivities and mechanisms of dietary proanthocyanidins on blood pressure lowering: A critical review of in vivo and clinical studies

Proanthocyanidins, widespread in natural plant sources, are bioactive substances that exhibit broad benefits to human health. Of note, proanthocyanidins have been reported to lower blood pressure and prevent hypertension, but a critical review of this is lacking. In this review, information on the basic structures and absorption of dietary proanthocyanidins as well as their bioactivities and related mechanisms on the lowering of blood pressure derived via in vivo and clinical studies are summarized. Clinical studies have shown that proanthocyanidins have a pronounced blood pressure-lowering effect, effectively preventing hypertension and reducing the occurrence of cardiovascular and cerebrovascular diseases. The potential mechanisms, which are herein reviewed in detail, involve the improvement of vascular function, reduction of oxidative stress and inflammation, and modulation of lipid metabolism. Taken together, this work provides information for a better understanding of the antihypertensive effects of proanthocyanidins, which may promote their use to reduce the risk of developing hypertension.

Purification and Characterization of a Dark Red Skin Related Dimeric Polyphenol Oxidase from Huaniu Apples

The distinct dark-red skin of Huaniu apples renders them attractive to customers. However, the mechanism that leads to the development of the color of the fruit is unclear. In this study, we found that compared with red Fuji (a bright-red apple cultivar), Huaniu apples had higher contents of (−)-epicatechin (EC), (−)-epigallocatechin (EGC), (−)-gallocatechin gallate (GCG), and procyanidins (PCs) B2 and C1 in the peel, which implies that the polymerization of the flavanols and PCs may be correlated with the dark-red skin of the fruit. Using EC as a substrate, we purified an enzyme from Huaniu peel. We performed protein sequencing and discovered that the enzyme was a polyphenol oxidase (PPO). The molecular weight of the enzyme was approximately 140 kDa, which we estimated by native-PAGE and SDS-PAGE, while it was 61 kDa by urea-SDS-PAGE, from which we discovered that the PPO was a dimer. We observed the lowest K_m value for catechol (0.60 mM), and the best substrate was 4-methylcatechol, with a V_max of 526.32 U mg⁻¹ protein. EC is a suitable natural substrate, with a K_m value of 1.17 mM, and 55.27% of the V_max/K_m of 4-methylcatechol. When we used EC as a substrate, the optimum temperature and pH of the PPO were 25 °C and 5.0, respectively. In summary, we purified a dimeric PPO from Huaniu apples that showed high activity to EC, which might catalyze the polymerization of flavanols and PCs and lead to the dark-red color development of the fruit.

Improving the color and functional properties of seabuckthorn seed protein with phytase treatment combined with alkaline solubilization and isoelectric precipitation

BACKGROUND

Reducing anti-nutritional factors like phytates in seed protein products requires an ongoing effort. This study was the first to investigate the phytic acid content in seabuckthorn seed protein (SSP) and its reduction by an exogenous phytase during protein isolation from seabuckthorn seed meal through the common alkaline solubilization-isoelectric precipitation process.

RESULTS

The additional phytase treatment could reduce the content of phytic acid from 22.46 to 13.27 g kg^-1 , leading to SSP products with lighter color (lower ΔE^* ), higher protein solubility, higher in vitro digestibility, but lower phenolic antioxidant content (including flavonoids and procyanidins) and some beneficial ions like Ca, Fe, Mg, and Zn. The Fourier transform infrared (FTIR) results indicated that the secondary structure of protein changed under the treatment with phytase. Correlation analysis showed that L* was significantly negatively correlated with TP, TPC and TF (P < 0.001), while a* and b* were significantly positively correlated with them (P < 0.001).

CONCLUSIONS

There may be a trade-off between protein functionalities and other health-promoting components when a phytase treatment is included in SSP isolation. © 2021 Society of Chemical Industry.

Laccase-Mediated Flavonoid Polymerization Leads to the Pericarp Browning of Litchi Fruit

Litchi pericarp turns brown rapidly after fruit harvest, while the mechanism remains obscure. The contents of (-)-epicatechin (EC) and procyanidins (PCs) A2/B1/B2/C1 decreased during the pericarp browning, and a previously identified laccase (ADE/LAC) showed activity to these compounds, with brown products observed in the reactions. By UPLC-DAD-QTOF-MS/MS, isomers of dimeric, trimeric, and tetrameric PCs were detected in the EC-ADE/LAC reaction. In the presence of cyanidin-3-O-rutiside and rutin, anthocyanin-EC and rutin-EC adducts were, respectively, produced, and darker brown precipitation was observed in these reactions relatively to the EC-ADE/LAC reaction alone. ADE/LAC catalyzed the conversion of PC B2 to A-type PC dimers and B-type PC tetramers. ADE/LAC complemented the transparent testa of Arabidopsis LAC15-loss-of-function mutant (tt10) to wild-type dark brown seed coat. The results demonstrated that ADE/LAC-mediated flavonoid polymerization played an important role in the browning of pericarp.

Role of Grape-Extractable Polyphenols in the Generation of Strecker Aldehydes and in the Instability of Polyfunctional Mercaptans during Model Wine Oxidation

Polyphenolic fractions from Garnacha, Tempranillo, and Moristel grapes were reconstituted to form model wines of identical pH, ethanol, amino acid, metal, and varietal polyfunctional mercaptan (PFM) contents. Models were subjected to a forced oxidation procedure at 35 °C and to an equivalent treatment under strict anoxia. Polyphenolic profiles significantly determined oxygen consumption rates (5.6-13.6 mg L^-1 day^-1), Strecker aldehyde (SA) accumulation (ratios max/min around 2.5), and levels of PFMs remaining (ratio max/min between 1.93 and 4.53). By contrast, acetaldehyde accumulated in small amounts and homogeneously (11-15 mg L^-1). Tempranillo samples, with highest delphinidin and prodelphinidins and smallest catechin, consume O₂ faster but accumulate less SA and retain smallest amounts of PFMs under anoxic conditions. Overall, SA accumulation may be related to polyphenols, producing stable quinones. The ability to protect PFMs as disulfides may be negatively related to the increase in tannin activity, while pigmented tannins could be related to 4-methyl-4-mercaptopentanone decrease.

Characterization of Natural and Alkaline-Oxidized Proanthocyanidins in Plant Extracts by Ultrahigh-Resolution UHPLC-MS/MS

In this study, we analyzed the proanthocyanidin (PA) composition of 55 plant extracts before and after alkaline oxidation by ultrahigh-resolution UHPLC-MS/MS. We characterized the natural PA structures in detail and studied the sophisticated changes in the modified PA structures and the typical patterns and models of reactions within different PA classes due to the oxidation. The natural PAs were A- and B-type PCs, PDs and PC/PD mixtures. In addition, we detected galloylated PAs. B-type PCs in different plant extracts were rather stable and showed no or minor modification due to the alkaline oxidation. For some samples, we detected the intramolecular reactions of PCs producing A-type ether linkages. A-type PCs were also rather stable with no or minor modification, but in some plants, the formation of additional ether linkages was detected. PAs containing PD units were more reactive. After alkaline oxidation, these PAs or their oxidation products were no longer detected by MS even though a different type and/or delayed PA hump was still detected by UV at 280 nm. Galloylated PAs were rather stable under alkaline oxidation if they were PC-based, but we detected the intramolecular conversion from B-type to A-type. Galloylated PDs were more reactive and reacted similarly to nongalloylated PDs.

Modification of Natural Proanthocyanidin Oligomers and Polymers Via Chemical Oxidation under Alkaline Conditions

We tested the susceptibility of 102 proanthocyanidin (PA)-rich plant extracts to oxidation under alkaline conditions and the possibility to produce chemically modified PAs via oxidation. Both the nonoxidized and the oxidized extracts were analyzed using group-specific ultrahigh-performance liquid chromatography-diode array detection-tandem mass spectrometry (UHPLC-DAD-MS/MS) methods capable of detecting procyanidin (PC) and prodelphinidin (PD) moieties along the two-dimensional (2D) chromatographic fingerprints of plant PAs. The results indicated different reactivities for PCs and PDs. When detected by UHPLC-DAD only, most of the PC-rich samples exhibited only a subtle change in their PA content, but the UHPLC-MS/MS quantitation showed that the decrease in the PC content varied by 0-100%. The main reaction route was concluded to be intramolecular. The PD-rich and galloylated PAs showed a different pattern with high reductions in the original PA content by both ultraviolet (UV) and MS/MS quantitation, accompanied by the shifted retention times of the chromatographic PA humps. In these samples, both intra- and intermolecular reactions were indicated.