>Magister Scientiae - MSc / Tyrosinase is an enzyme widely distributed in the biosphere and is found in many species of bacteria, fungi, animals, and plants; it is associated with melanin production. Even though it possesses many beneficial properties such as photoprotection, but overproduction causes undesirable effects such melasma, solar lentigines etc. Therefore, tyrosinase enzyme inhibitors are of far-ranging importance in cosmetics, medicinal products, and food industries. This study is aimed to test anti-tyrosinase activity in 37 plants from 20 families using mushroom tyrosinase inhibition method; each plant was extracted with methanol. The results showed that 17 plant extracts, exerted a considerable level of in vitro tyrosinase inhibition comparable to positive controls of kojic acid in the same solvent systems when evaluated spectrophotometrically. Among plant extracts, those that showed an inhibition rate >50 % at 50 μg/ml and ˃60 % at 200 μg/ml were A. karroo (Hayne.), A. afra Jacq. Ex Willd, C. geifolia (L.), E. racemosa (L.), H. petiolare Hilliard & B.L.Burt, M. quercifolia (L.), M. communis (L.), P. rigida (Wikstr.), P. ecklonii (Benth.), P. ericoides (L.), S. Africanacaerulea (L.), S. Africana-lutea (L.), S. antarcticus (Willd.), S. lucida (L.) F.A.Barkley, S. hamilifolius (L.), S. furcellata R.Br and T riparia which exhibited great anti-tyrosinase activity.

Magister Scientiae - MSc / Various dermatological disorders, such as formation of black pigmented patches on the surface of the skin arise from the over-activity of tyrosinase enzyme's degenerative action. This enzyme is further implicated in the involvement of melanin in malignant melanoma, the most lifethreatening skin tumors. Although, synthetic products were found effective to combat this menace, nevertheless, overtime detrimental effect on human skin is a challenge. Investigation of natural tyrosinase inhibitors from methanol extracts of medicinal plants of Lamiaceae family using L-tyrosine as substrate on three different complementary assays (TLC bio-autography, spectrophotometry and cyclic voltammetry) was carried out accordingly. The result indicated Salvia chamelaeagnea, Salvia dolomitica, Plectranthus ecklonii, Plectranthus namaensis, and Plectranthus zuluensis, with significant zone of inhibition against tyrosinase on TLC bio-autography, spectrophotometry result showed that extracts of Plectranthus ecklonii (IC50 = 21.58 μg/mL), Plectranthus zuluensis (IC50 = 23.99 μg/mL), Plectranthus madagascariensis (IC50 = 23.99 μg/mL) and Salvia lanceolata (IC50 = 28.83) demonstrated good anti-tyrosinase activity when compared with kojic acid (IC50 = 3.607 μg/mL). On the other hand, cyclic voltammetry are in consonant with above results thereby supported the nomination of some of the extracts as strong anti-tyrosinase agents. Salvia chamelaeagnea showed strong activity in cyclic voltammetry and clear zone of inhibition on TLC bioautography, these reasons gave us justification for further chemical study to isolate the bioactive constituents. Phytochemical investigation of the bioactive extract of Salvia chamelaeagnea using different chromatographic methods including column chromatographic and semi preparative HPLC afforded six (6) known compounds viz carsonol (C1), carnosic acid (C2), 7- ethoxylrosmanol (C3), ursolic acid (C4), rosmanol (C5) and ladanein (C6). Their chemical structures were elucidated by analyses of spectroscopic (1H and 13C NMR) data as well as correlations with existing literature. The methanolic extract of S. chamelaeagnea (SC) showed moderate antityrosinase (IC50 = 267.4 μg/mL) activity, total antioxidant capacities measured as: Oxygen radicals absorbance capacity (ORAC; 14970 ± 5.16 μM TE/g), ferric-ion reducing antioxidant power (FRAP; 9869.43 ± 7.87 μM AAE/g) and trolox equivalent absorbance capacity (TEAC; 13706.5 ± 0.95 μM TE/g). Excellent total antioxidant capacities were demonstrated by C1 and C5 respectively as FRAP (9338.92 ± 1.72; 8622.73 ± 1.92) μM AAE/g; TEAC (16505 ± 0.86; 10641.5 ± 0.52) μM TE/g; ORAC (14550.5 ± 3.65; 14633.90 ± 3.84) μM TE/g and including the inhibition of Fe2+ -induced lipid peroxidation (IC50 = 32.5; 30.25) μg/mL. All the compounds except C4 are electro-active with well-defined oxidation-reduction peaks while C1 demonstrated the highest tyrosinase inhibitory activity by strongly decreased the inhibition current with time using cyclic voltammetry method. The isolated compounds especially C1, C2 and C5 are well known to combat with ageing problems and documented for their powerful activity against oxidative stress and alzheimer's diseases, which are ageing related symptoms. The isolation of such bioactive compounds indicated the synergetic effect of the results of the three methods used in this thesis. This is the first report on the evaluation of both anti-tyrosinase and total antioxidant capacities of the isolated compounds from S. chamelaeagnea. The findings therefore can be used as background information for exploitation of skin depigmentation and antioxidant agents from natural source.