Characterization of particulate D-apiosyl-and D-xylosyltransferase from Lemna minor.

A particulate enzyme preparation capable of catalyzing the transfer of d-U-¹⁴C]apiose and d-U-¹⁴C]xylose from uridine 5′-(α-d-U-¹⁴C]apio-d-furanosyl pyrophosphate) (UDPU-¹⁴C]Api) and uridine 5′-(α-d-U-¹⁴C]xylopyranosyl pyrophosphate) (UDPU-¹⁴C]Xyl) to endogenous acceptor molecules was isolated from Lemna minor. The two enzymes were named UDP-d-apiose:acceptor d-apiosyltransferase and UDP-d-xylose:acceptor d-xylosyltransferase and were associated with particulate material sedimenting between 480 and 34,800g. The rate of d-U-¹⁴C]apiose or d-U-¹⁴C]xylose incorporation was proportional to the quantity of enzyme preparation used and was constant with time to 1.5 min. Both enzymes showed a pH optimum of 5.7 in citrate-phosphate buffer. The d-apiosyltransferase has a K_m for UDPU-¹⁴C]Api of 4.9 μm. Bovine serum albumin and sucrose stimulated the rate of incorporation of both pentoses. Both enzymes rapidly lost activity; with our best conditions, approximately 50% of each enzyme activity was lost in 6 min at 25 °C or in 3 h at 4 °C. Incorporation of d-U-¹⁴C]apiose was obtained in the absence of added uridine 5′-(α-d-galactopyranosyluronic acid pyrophosphate) (UDPGalUA); however, the addition of UDPGalUA not only almost doubled the rate of incorporation, but also increased the total incorporation of d-U-^l4C]apiose and extended the proportional range of incorporation at 25 °C from 1.5 to 2 min.