Melanoidins From Prunus mume Fruit-Juice Concentrate: Polarity-Driven Differences in Immunomodulatory Activity

Melanoidins, a key component of Prunus mume fruit-juice concentrate (PFC), have gained increasing attention due to their health-promoting potency. However, the physicochemical properties of melanoidins in PFC and their biological activities remain poorly understood, which impedes the comprehensive utilization of PFC's functional value. In this study, three melanoidins fractions (PFCM1, PFCM2, and PFCM3) with distinct polarities were isolated using macroporous resin by sequential elution with water, 20% ethanol, and 50% ethanol. With increasing polarity (PFCM1 > PFCM2 > PFCM3), the total contents of carbohydrates, phenolics, and protein decreased inversely (PFCM3 > PFCM2 > PFCM1), the levels of total monosaccharides increased, and total amino acids of the melanoidins decreased. NMR analysis indicated that the degree of unsaturation of the backbone increased as the polarity of melanoidins decreased. Furthermore, the antioxidant activity and Fe²⁺ chelating ability of three melanoidins showed a negative correlation with their polarity. All fractions promoted the phagocytosis of RAW264.7 cells and significantly (p < 0.05) enhanced the cellular secretion of NO as well as cytokines TNF-α, IL-1β, and IL-6, exhibiting an immune-activating effect. Among them, PFCM3 exhibited the most pronounced immunomodulatory effects. Transcriptomic and Western blot analyses revealed that the effects of PFCM3 on RAW264.7 cells were primarily exerted through the immune system, with the TLR2/4-mediated MAPK pathway representing the primary signaling pathway involved in its immunomodulatory activity. Our findings suggest that the melanoidins from PFC with varying polarity exhibited distinct immunoregulatory ability, which can be attributed to the differences in their structural features. PRACTICAL APPLICATIONS: Melanoidins are ubiquitously distributed in food matrices and have exhibited multiple bioactivities. However, these compounds have received limited attention during the biofunctional evaluation of certain traditional functional foods. This study demonstrated that melanoidins, as a significant component in Prunus mume fruit-juice concentrate (PFC), exert immune activation effects through membrane receptor-mediated MAPK signaling pathway. Moreover, their immunomodulatory capability is correlated with the polarity of melanoidins, a property associated with their distinct structural features. This study deepens our understanding of the health benefits of foods rich in melanoidins and provides valuable insight into the relationship between structural features and bioactivities of melanoidins.