Excretion of laccase by sycamore (Acer pseudoplatanus L.) cells. Purification and properties of the enzyme

A laccase-type polyphenol oxidase is excreted by sycamore cells (Acer pseudoplatanus L.) cells. The enzyme has been purified by classical purification techniques. It is a blue copper protein of Mr 97 000, containing 45% carbohydrate and 0.24% copper. This protein consists of one single unit and the copper content corresponds to four copper atoms per protein molecule. The specific activity of the purified extracellular sycamore-cell laccase measured at pH 6.6 (optimum pH) and in the presence of 20mM-4-methhylcatechol (optimum substrate conditions) corresponded to an oxygen uptake of 32 000 nmol of O2/min per mg of protein. Under these conditions, the catalytic-centre activity of the enzyme reached 100 s-1. The excretion of laccase by sycamore cells is significant, being about 2% of the total protein synthesized by the cells during the exponential phase of growth, and is independent of cell growth. The physiological significance and the problems raised by the passage of this protein across the cytoplasmic membrane are discussed.