Two a-i-rhamnohydrolases with different substrate specificities were isolated from a commercial preparation produced by Aspergillus aculeatus. l h e first rhamnohydrolase was active toward pnitrophenyl-a-i-rhamnopyranoside, naringin, and hesperidin and was termed p-nitrophenyl-a-i-rhamnopyranohydrolase (pnprhamnohydrolase). From the data collected, the enzyme seemed specific for the a-1,2-or a-1,6-linkage to B-D-glucose. l h e pnprhamnohydrolase had a molecular mass of 87 kD (sodium dodecyl sulfate-polyacrylamide gel electrophoresis), a pH optimum of 5.5 to 6, a temperature optimum of 60°C, and a specific activity toward pnp-a-i-rhamnopyranoside (pnp-Rha) of 13 units mg-' protein.l h e second rhamnohydrolase, on the contrary, was active toward rhamnogalacturonan (RC) fragments, releasing Rha, and was therefore termed RC-rhamnohydrolase. l h e RC-rhamnohydrolase had a molecular mass of 84 kD, a pH optimum of 4, a temperature optimum of 60'C, and a specific activity toward RC oligomers of 60 units mg-' protein. l h e RC-rhamnohydrolase liberated Rha from the nonreducing end of the RC chain and appeared specific for the a-1,4-linkage to a-D-galacturonic acid. l h e enzyme was hindered when this terminal Rha residue was substituted at the 4-position by a 8-D-galactose. The results so far obtained did not indicate particular preference of the enzyme for low or high molecular mass RC fragments. From the results it can be concluded that a new enzyme, an RC a-i-rhamnopyranohydrolase, has been isolated with high specificity toward RC regions of pectin.

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l h e recently described rhamnogalacturonase B, which is able to degrade ramified hairy regions of pectin, was found to be a rhamnogalacturonan a-i-rhamnopyranosyl-( 1 +4)-a-~-galactopyranosyluronide lyase. The cleavage site and mechanism differ from that of the previously described rhamnogalacturonase A, which is a hydrolase and can now be termed rhamnogalacturonan a-D-galactopyranosyluronide-( 1 +2)-a-i-rhamnopyranosyl hydrolase.

The maximum reaction rate of rRG-lyase toward MHR-S at pH 6.0 and 31°C was 28 units mg ؊1 . rRG-lyase and RG-hydrolase cleave the same alternating RG I subunit in MHR. Both of these enzymes fragment MHR by a multiple attack mechanism. The catalytic efficiency of rRG-lyase for MHR increases with decreasing degree of acetylation. Removal of arabinose side chains improves the action of rRG-lyase toward MHR-S. In contrast, removal of galactose side chains decreased the catalytic efficiency of rRG-lyase. Native RGlyase was purified from A. aculeatus, characterized, and found to be similar to the rRG-lyase expressed in Aspergillus oryzae.

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