Alpha-Glucosidase and Alpha-Amylase Inhibitory Activities of Novel Abietane Diterpenes from Salvia africana-lutea

Abstract

The re-investigation of a methanolic extract of <i>Salvia africana-lutea</i> collected from the Cape Floristic Region, South Africa (SA), afforded four new abietane diterpenes, namely 19-acetoxy-12-methoxycarnosic acid (<b>1</b>), 3β-acetoxy-7α-methoxyrosmanol (<b>2</b>), 19-acetoxy-7α-methoxyrosmanol (<b>3</b>), 19-acetoxy-12-methoxy carnosol (<b>4</b>), and two known named clinopodiolides A (<b>5</b>), and B (<b>6</b>), in addition to four known triterpenes, oleanolic, and ursolic acids (<b>7</b>, <b>8</b>), 11,12-dehydroursolic acid lactone (<b>9</b>) and β-amyrin (<b>10</b>). The chemical structural elucidation of the isolated compounds was determined on the basis of one and two dimensional nuclear magnetic resonance (1D and 2D NMR), high-resolution mass spectrometry (HRMS), ultra violet (UV), fourier transform infrared (IR), in comparison with literature data. The in vitro bio-evaluation against alpha-glucosidase showed strong inhibitory activities of <b>8</b>, <b>10</b>, and <b>7</b>, with the half inhibitory concentration (IC₅₀) values of 11.3 ± 1.0, 17.1 ± 1.0 and 22.9 ± 2.0 µg/mL, respectively, while <b>7</b> demonstrated the strongest in vitro alpha-amylase inhibitory activity among the tested compounds with IC₅₀ of 12.5 ± 0.7 µg/mL. Additionally, some of the compounds showed significant antioxidant capacities. In conclusion, the methanolic extract of <i>S. africana-lutea</i> is a rich source of terpenoids, especially abietane diterpenes, with strong antioxidant and anti-diabetic activities that can be helpful to modulate the redox status of the body and could therefore be an excellent candidate for the prevention of the development of diabetes, a disease where oxidase stress plays an important role.