Why is biotin starvation necessary for the excretion of glutamic acid? The major function of biotin is to serve as a prosthetic group in acetyl-CoA car- boxylase, the first enzyme of fatty acid synthesis, so it was hypothesized that the excretion might be related to an increased general permeability of the cell membrane, caused by an insufficiency of fatty acids. This notion appeared to be supported by the finding that other treatments affecting the integrity of the cell membrane