Melanin is the pigment that is responsible for the colour of eyes, hair, and skin in humans. Tyrosinase is known to be the key enzyme
in melanin biosynthesis. Overactivity of this enzyme leads to dermatological disorders such as age spots, melanoma and sites of
actinic damage. Ten plants belonging to four families (Asphodelaceae, Anacardiaceae, Oleaceae, and Rutaceae) were investigated
for their effect on tyrosinase using both L-tyrosine and L-DOPA as substrates. Ethanol leaf extracts (500 μg/mL) of Aloe ferox,
Aloe aculeata, Aloe pretoriensis, and Aloe sessiliflora showed 60%, 31%, 17%, and 13% inhibition of tyrosinase activity respectively,
when L-tyrosine was used as a substrate. Harpephyllum caffrum (leaves) at a concentration of 500 μg/mL had an inhibitory effect
of 70% on tyrosinase when L-DOPA was used as a substrate. The IC50 of Harpephyllum caffrum (leaves and bark) were found to be
51±0.002 and 40±0.035 μg/mL, respectively. Following the results obtained fromthe tyrosinase assay, extracts from Harpephyllum
caffrum were selected for further testing on their effect on melanin production and their cytotoxicity on melanocytes in vitro.
The IC50 of both extracts was found to be 6.25 μg/mL for melanocyte cells. Bark extract of Harpephyllum caffrum showed 26%
reduction in melanin content of melanocyte cells at a concentration of 6.25 μg/mL. The leaf extract of this plant showed some
toxicity on melanocyte cells. Therefore, the bark extract of Harpephyllum caffrum could be considered as an antityrosinase agent
for dermatological disorders such as age spots and melasoma.