Medic Aljaz, Zamljen Tilen, Hudina Metka, Veberic Robert
Department of Agronomy, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, SI-1000 Ljubljana, Slovenia.
Biology (Basel). 2022 Feb 21;11(2):342. doi: 10.3390/biology11020342.
The aim of the present study was to investigate how individual phenolic compounds and phenolic groups in walnut husk gratings (e.g., naphthoquinones, flavanols, flavonols, hydroxycinnamic acids) are oxidized over time, with a particular focus on the juglone synthase pathway. Walnut husk gratings were prepared and left under 'degradation' conditions (exposure to the air, room temperature) at increasing times. Following methanol extraction of these husk gratings, the HPLC profile of methanolic extract of husk gratings exhibited twenty-six compounds over time, then hydrojuglone glucoside, α-hydrojuglone, and juglone were detailed by HPLC-mass spectrometry. Initially (0-20 min), the content of hydrojuglone glucoside in the husk gratings decreased by 40.4%, while the content of α-hydrojuglone increased by 20.0%, and then decreased. After an initial delay (0-20 min), juglone increased by 47.9% from 20 to 40 min, and then decreased. This initially confirmed that hydrojuglone glucoside and α-hydrojuglone could be considered as precursors of juglone. Different phenolic groups showed different degradation processes, although they all reached their highest content after 40 min. This might arise from degradation of the phenols, increased free phenols, or activation of the plant defense mechanism due to damage to the tissue, similar to the effects of stress or a pathogen attack. Although it has been reported that the phenolic compounds decrease when food is processed or damaged, they showed increases, which were not indefinite, but time dependent. As phenolic compounds are considered highly beneficial to human health, increases upon processing indicate the need for further investigations into healthier food preparation processes. This is the first study on the degradation pathways of juglone, using a mass spectrometer, in which we suggest that hydrojuglone glucoside and α-hydrojuglone are indeed the precursors of juglone. However, it is possible that there are other degradation pathways of hydrojuglone glucoside, since less juglone is synthesized than expected.
本研究的目的是调查核桃壳格栅中的单个酚类化合物和酚类基团（如萘醌、黄烷醇、黄酮醇、羟基肉桂酸）如何随时间氧化，特别关注胡桃醌合酶途径。制备核桃壳格栅，并在“降解”条件下（暴露于空气中，室温）放置不同时间。对这些壳格栅进行甲醇提取后，壳格栅甲醇提取物的HPLC图谱随时间显示出26种化合物，然后通过HPLC-质谱详细分析了氢化胡桃醌葡萄糖苷、α-氢化胡桃醌和胡桃醌。最初（0-20分钟），壳格栅中氢化胡桃醌葡萄糖苷的含量下降了40.4%，而α-氢化胡桃醌的含量增加了20.0%，然后下降。经过初始延迟（0-20分钟）后，胡桃醌在20至40分钟内增加了47.9%，然后下降。这初步证实了氢化胡桃醌葡萄糖苷和α-氢化胡桃醌可被视为胡桃醌的前体。不同的酚类基团显示出不同的降解过程，尽管它们在40分钟后都达到了最高含量。这可能是由于酚类物质的降解、游离酚类物质增加或组织受损导致植物防御机制激活，类似于应激或病原体攻击的影响。尽管有报道称，食品加工或受损时酚类化合物会减少，但它们却出现了增加，这种增加并非无限期的，而是与时间有关。由于酚类化合物被认为对人体健康非常有益，加工过程中的增加表明需要进一步研究更健康的食品制备过程。这是第一项使用质谱仪研究胡桃醌降解途径的研究，我们认为氢化胡桃醌葡萄糖苷和α-氢化胡桃醌确实是胡桃醌的前体。然而，由于合成的胡桃醌比预期的少，氢化胡桃醌葡萄糖苷可能还有其他降解途径。
