Analysis of the ACP1 gene product: classification as an FMN phosphatase

The relationship between the ACP1 gene product, an 18kDa acid phosphatase (E.C. 3.1.3.2) postulated to function as a protein tyrosyl phosphatase, and the cellular flavin mononucleotide (FMN) phosphatase has been examined in vitro and by using cultured Chinese hamster ovary (CHO) cells. Kinetic analysis indicated that at pH 6 the acid phosphatase utilized a variety of phosphate monoesters as substrates. While small molecules such as FMN were effectively utilized as substrates (kcat/Km = 7.3 x 10(3) s-1M-1), the tyrosyl phosphorylated form of the adipocyte lipid binding protein was a relatively poor substrate (kcat/Km = 1.7 x 10(-1) s-1M-1) suggesting a role for the phosphatase in flavin metabolism. Fractionation of CHO cell extracts revealed that 90% of the FMN phosphatase activity was soluble and that all of the soluble activity eluted from a Sephadex G-75 column with the acid phosphatase. All of the soluble FMN phosphatase activity was inhibited by immunospecific antibodies directed against the bovine heart ACP1 gene product. These results suggest that the ACP1 gene product functions cellularly not as a protein tyrosyl phosphatase but as a soluble FMN phosphatase.