try this protocol.... works well in our lab.. Foyer CH, Rowell J, Walker D (1983) Measurement of the ascorbate content of spinach leaf protoplasts and chloroplasts during illumination. Planta 157:239–244
The protocol works well with protoplasts from plant cells also. so in general for " tissue or cell extracts " its would not be bad to give it a try as its pretty simple one.