Determination of PQQ-adduct with PQQ-liberating enzyme

通过将PQQ与丝氨酸孵育，形成两种PQQ-丝氨酸。这些PQQ-丝氨酸的比例取决于反应混合物的pH。当PQQ与丝氨酸在pH 8.0下形成加合物时，得到最简单的PQQ-恶唑，PQQ-甘氨酸作为唯一的PQQ-加合物。另一方面，当在pH 3.5下孵育时，形成另一种PQQ-加合物。这两种PQQ-加合物本身是稳定的，并且彼此不能相互转化。结构分析结果表明，在pH3.5条件下制备的PQQ-丝氨酸具有一个来源于丝氨酸的-CH_2OH基团。当PQQ与各种氨基酸在pH3.5条件下反应时，只生成一种PQQ-加合物，其化学结构与PQQ-甘氨酸不同。两种PQQ-丝氨酸均表现出明显的微生物生长刺激活性。在明确限定的条件下清楚地观察到大肠杆菌。这种PQQ效应被归类为II型效应。当E.大肠杆菌在含PQQ的培养基上生长，约50%的GDH在培养过程中转化为全酶。此时，E。大肠杆菌消耗大量的葡萄糖，并且培养基比不含PQQ的培养基高度酸化。根据这些结果，GDH将葡萄糖氧化为葡萄糖酸盐，但葡萄糖酸盐很少被细胞吸收。GDH的功能是产生质子梯度，然后合成ATP。ATP通过PTS支持葡萄糖摄取，随后支持E.大肠杆菌受到刺激。Apo-GDH广泛分布于微生物中，因此，这种PQQ效应估计是一种普遍现象。本文还讨论了PQQ及其加合物对微生物生长刺激作用的机理。

Two species of PQQ-serine were formed by incubating PQQ with serine. The ratio of these PQQ-serine was depend on pH of the reaction mixture. When PQQ-adducts formation with serine was conducted at pH 8.0, the simplest PQQ-oxazole, PQQ-glycine was given as sole PQQ-adduct. On the other hand, another species of PQQ-adduct was formed when incubated at pH 3.5. These two PQQ-adducts were stable themselves and were not interconvertible each other. As a result of structural analysis, the PQQ-serine prepared at pH 3.5 has a -CH_2OH group originated from serine. When the reaction of PQQ with various amino acids was carried out at pH 3.5, only one species of PQQ-adduct was formed, which are different in chemical structure from PQQ-glycine. Both species of PQQ-serine exhibited a marked growth stimulating activity for microorganisms.Growth stimulating effect of PQQ for E. coli was clearly observed under well defined conditions. This PQQ effect was classfied to the type II effect. When E. coli grew on a medium containing PQQ, about 50% of whole GDH was converted to holoenzyme during cultivation. At this moment, E. coli consumed a large amount of glucose and the medium was highly acidified more than in a medium excluding PQQ. From these results, GDH oxidizes glucose to gluconate but gluconate was scarecely taken up into the cell. GDH functions to generate proton gradient, followed by ATP synthesis. The ATP supports glucose uptake via PTS, subsequently the growth of E. coli is stimulated. Apo-GDH is widely distributed in microorganisms, thus, this PQQ effect is estimated to be an universal phenomenon. A proposed mechanism of growth stimulating effect of PQQ and PQQ-adducts for microorganism is discussed.