Fine Structure of SMG Alginate Fragment in the Light of Its Degradation by Alginate Lyases of Pseudomonas sp.

An alginate fragment named SMG, consisting of mannuronic (M) and guluronic acid residues (G) ( $DP¯$ =25), was prepared from the partial acid hydrolysate of a commercial alginate. Two subfractions, SMG-ppt ( $DP¯$ =52) and SMG-sup ( $DP¯$ =18) were obtained from SMG by fractionation with MgCl₂ and CaCl₂. The M/G ratios of these alginate fragment were 1.4–1.9. Their lysis products by a pseudomonad alginate lyase [EC 4.2.2.3] preparation were fractionated by gel filtration, giving similar patterns. The major products in their digests were unsaturated monouronides (53–50%) and triuronides (30–35%). The former was identified as a δ4,5-hexuronic acid (δU) and the latter was identified as a mixture of δ4,5-hexuronosyl-(1→4)-β-D-mannuronosyl-(1→4)-L-guluronic acid (δUMG) and δ4,5-hexuronosyl-(1→4)-α-L-guluronosyl-(1→4)-L-guluronic acid (δUGG). The two unsaturated triuronides were present in roughly equal amounts. The presence of 4-O-α-L-guluronosyl-L-guluronic acid (GG) and 4-O-β-D-mannuronosyl-L-guluronic acid (MG) or 4-O-β-L-guluronosyl-D-mannuronic acid (GM) was also demonstrated in the digest. Moreover, indirect evidence suggested nonreducing terminal δU residue and free δU in the digest to be derived more from M than G of the original SMG. Thus, it was concluded that more than one-third of uronic acid residues of SMG molecules may be composed of almost equal amounts of MG and GG sequences, most of which may be connected with M to form MMG and MGG sequences, respectively.