RHAMNOGALACTURONAN ALPHA-L-RHAMNOPYRANOHYDROLASE - A NOVEL ENZYME SPECIFIC FOR THE TERMINAL NONREDUCING RHAMNOSYL UNIT IN RHAMNOGALACTURONAN REGIONS OF PECTIN

Two alpha-L-rhamnohydrolases with different substrate specificities we re isolated from a commercial preparation produced by Aspergillus acul eatus. The first rhamnohydrolase was active toward p-nitrophenyl-alpha -L-rhamnopyranoside, naringin, and hesperidin and was termed p-nitroph enyl-alpha-L-rhamnopyranohydrolase (pnp-rhamnohydrolase). From the dat a collected, the enzyme seemed specific for the alpha-1,2- or alpha-1, 6-linkage to beta-D-glucose. The pnp-rhamnohydrolase had a molecular m ass of 87 kD (sodium dodecyl sulfate-polyacrylamide gel electrophoresi s), a pH optimum of 5.5 to 6, a temperature optimum of 60 degrees C, a nd a specific activity toward pnp-alpha-L-rhamnopyranoside (pnp-Rha) o f 13 units mg(-1) protein. The second rhamnohydrolase, on the contrary , was active toward rhamnogalacturonan (RG) fragments, releasing Rha, and was therefore termed RG-rhamnohydrolase. The RG-rhamnohydrolase ha d a molecular mass of 84 kD, a pH optimum of 4, a temperature optimum of 60 degrees C, and a specific activity toward RG oligomers of 60 uni ts mg(-1) protein. The RG-rhamnohydrolase liberated Rha from the nonre ducing end of the pc chain and appeared specific for the alpha-1,4-lin kage to alpha-D-galacturonic acid. The enzyme was hindered when this t erminal Rha residue was substituted at the 4-position by a beta-D-gala ctose. The results so far obtained did not indicate particular prefere nce of the enzyme for low or high molecular mass RG fragments. From th e results it can be concluded that a new enzyme, an RG alpha-L-rhamnop yranohydrolase, has been isolated with high specificity toward RG regi ons of pectin.