Mammalian intestinal alkaline phosphatase acts as highly active exopolyphosphatase

Recent results revealed that inorganic polyphosphates (polyP), being energy -rich linear polymers of orthophosphate residues known from bacteria and ye ast, also exist in higher eukaryotes. However, the enzymatic basis of their metabolism especially in mammalian cells is still uncertain. Here we demon strate for the first time that alkaline phosphatase from calf intestine (CI AP) is able to cleave polyP molecules up to a chain length of about 800, Th e enzyme acts as an exopolyphosphatase degrading polyP in a processive mann er. The pH optimum is in the alkaline range. Divalent cations are not requi red for catalytic activity but inhibit the degradation of polyP. The rate o f hydrolysis of short-chain polyP by CIAP is comparable to that of the stan dard alkaline phosphatase (AP) substrate p-nitrophenyl phosphate. The speci fic activity of the enzyme decreases with increasing chain length of the po lymer both in the alkaline and in the neutral pH range. The K, of the enzym e also decreases with increasing chain length. The mammalian tissue non-spe cific isoform of AP was not able to hydrolyze polyP under the conditions ap plied while the placental-type AP and the bacterial (Escherichia coli) AP d isplayed polyP-degrading activity. (C) 2001 Published by Elsevier Science B .V.