Hydride Transfer in the Biosynthesis of Uridine Diphospho‐Apiose from Uridine Diphospho‐ d ‐glucuronic Acid with an Enzyme Preparation of Lemna minor

The fate of the hydrogen atoms at C‐3 and C‐4 of uridine diphospho‐d‐glucuronic acid (UDP‐GlcUA) in the course of its conversion to uridine diphospho‐apiose (UDP‐api) was investigated. The specifically labeled UDP‐GlcUA substrates were prepared enzymatically from d‐[3‐3H]glucose and d‐[4‐3H]glucose by the combined action of hexokinase, phosphoglucomutase, UDPG‐pyrophosphorylase, and UDPG dehydrogenase. UDP‐[U‐14C, 3‐3H]GlcUA, UDP‐[U14C, 4‐3H]‐GlcUA, and UDP‐[14C, 3,4‐3H2]GlcUA were separatelly converted to UDP‐Api by an enzyme preparation from Lemna minor. In order to obtain an apiose (3‐C‐hydroxymethyl‐d‐erythro‐furanose) derivative which was reasonably stable and which could be produced without passing through the aldehydo form, the apiosyl radical of UDP‐Api was enzymatically transferred to 7‐glucosylapigenin by an enzyme preparation from parsley. The product of this reaction, apiin (7‐O‐[β‐d‐apiofuranosyl‐(1→2)‐β‐d‐glucosyl‐5,7,4′‐trihydroxyflavone), was subjected to mild periodate oxidation to convert the branch hydroxymethyl group of the apiosyl residue to formaldehyde. Approximately 14–16% of each of the substrates was converted to UDP‐Api. The transfer of the apiosyl moiety to 7‐glucosylapigenin was essentially quantitative in all cases. The isolated apiin was shown to have retained 53.5% of the tritium from the substrate labeled at C‐3, 99% from the substrate labeled at C‐4, and 73% from the substrate labeled at C‐3,4. All of the tritium and one‐fifth of the 14C of apiin produced from the variously labeled substrate was accounted for in the formaldehyde‐dimedone derivative. These results establish the occurrence of a hydride shift from C‐4 of UDP‐GlcUA to C‐3′ of apiose in the biosynthesis of UDP‐Api from UDP‐GlcUA.