Hizikia fusiformis (Harvey) Okamura is an edible marine alga that has been widely used in Korea, China, and Japan as a rich source of dietary fiber and essential minerals. In our previous study, we observed that the methanol extract of H. fusiformis and its non-polar fractions showed potent protein tyrosine phosphatase 1B (PTP1B) and α-glucosidase inhibition. Therefore, the aim of the present study was to identify the active ingredient in the methanol extract of H. fusiformis. We isolated a new glycerol fatty acid (13) and 20 known compounds including 9 fatty acids (1–3, 7–12), mixture of 24R and 24S-saringosterol (4), fucosterol (5), mixture of 24R,28R and 24S,28R-epoxy-24-ethylcholesterol (6), cedrusin (14), 1-(4-hydroxy-3-methoxyphenyl)-2-[2-hydroxy -4-(3-hydroxypropyl)phenoxy]-1,3-propanediol (15), benzyl alcohol alloside (16), madhusic acid A (17), glycyrrhizin (18), glycyrrhizin-6’-methyl ester (19), apo-9′-fucoxanthinone (20) and tyramine (21) from the non-polar fraction of H. fusiformis. New glycerol fatty acid 13 was identified as 2-(7′- (2″-hydroxy-3″-((5Z,8Z,11Z)-icosatrienoyloxy)propoxy)-7′-oxoheptanoyl)oxymethylpropenoic acid by spectroscopic analysis using NMR, IR, and HR-ESI-MS. We investigated the effect of the 21 isolated compounds and metabolites (22 and 23) of 18 against the inhibition of PTP1B and α-glucosidase enzymes. All fatty acids showed potent PTP1B inhibition at low concentrations. In particular, new compound 13 and fucosterol epoxide (6) showed noncompetitive inhibitory activity against PTP1B. Metabolites of glycyrrhizin, 22 and 23, exhibited competitive inhibition against PTP1B. These findings suggest that H. fusiformis, a widely consumed seafood, may be effective as a dietary supplement for the management of diabetes through the inhibition of PTP1B.
