Folic acid and pterin deaminases in Dictyostelium discoideum: kinetic properties and regulation by folic acid, pterin, and adenosine 3',5'-phosphate

Abstract

Kinetic data obtained for deamination of pterin by the extracellular fraction from Dictyostelium discoideum yielded apparently linear Lineweaver-Burk plots for pterin. The Michaelis constant for pterin was 30 ,uM. The data for folic acid deamination yielded convex Lineweaver-Burk plots. Convex Lineweaver-Burk plots could result from the presence of two types of enzymes with different affinities. The data for folic acid deamination were analyzed mathematically for two types of enzymes. This analysis produced Michaelis constants for folic acid of 1.8 and 23 ,iM. Competition studies suggested that an enzyme with low affinity nonspecifically catalyzed the deamination of folic acid and pterin, whereas an enzyme with high affinity was a specific folic acid deaminase. A specific folic acid deaminase with high affinity appeared to be present on the surface of D. discoideum cells. The Michaelis constant for this enzyme was 2.6 ,uM. Cells growing in nutrient broth and cells starved in phosphate buffer released folic acid and pterin deaminases. The quantity of deaminase activities released by the cells appeared to be controlled by chemoattractants. Starving cells that were supplied with folic acid, pterin, or adenosine 3',5'-phosphate increased their extracellular folic acid and pterin deaminase activities to a larger extent than did cell suspensions to which no chemoattractants were added. Administration of folic acid or pterin to starving cells caused increases of the activity of extracellular adenosine 3',5'phosphate phosphodiesterase and repressed increases of the activity of phosphodiesterase inhibitor.

Folate responsiveness during growth and development of Dictyostelium: separate but related pathways control chemotaxis and gene regulation

“…So far, the experimental evidence by Segall etal. (1988) and others (Wurster and Butz, 1983;deWit et ai, 1986;) strongly indicates Ihat the chemotaxis response is mediated by the N^°-methylfolate-sensitive B-sites of the receptor, which also confer the cGMP response. Both folate-stimulated cGMP production and folate chemotaxis are impaired in HG5145 but discoidin I regulation is unaffected.…”

Molecular basis of transmembrane signal transduction in Dictyostelium discoideum.

“…Various lines of evidence suggest that folates, pteridines, and cAMP are detected by different types of cell surface receptors (53,301,332); however, direct binding studies on pteridine receptors have not been done yet. Initial studies on the folic acid receptors were complicated by the high activity of the folic acid-degrading enzyme folate deaminase that is present on the surface of D. discoideum cells (13,330). This problem was solved by using the folate deaminase inhibitor 8azaguanine (59) or the degradation-resistant folic acid analog methotrexate (71,214).…”

“…The main difference between the A and B receptor types is that the A sites have much less specific requirements with respect to the structure of the pterine part of the folic acid molecule than the B sites (65). The binding specificities distinguish both A and B receptor types from each other and from the two folic acid-degrading enzymes present on cells, i.e., folate deaminase (13,330) and folate C9-N1O-cleaving enzyme (65,67).…”

A study on sensing and adaptation in Dictyostelium discoideum: guanosine 3',5'-phosphate accumulation and light-scattering responses.

“…zyme-catalyzed reactions. Cyclic AMP is hydrolyzed to 5'-AMP by means of cyclic AMP phosphodiesterase (for references see reference 10), and folio acid and pterin most likely are deaminated hydrolytically to the corresponding lumazine forms by means of folic acid and pterin deaminases (for references see references 10,11). Cyclic AMP phosphodiesterases as well as folic acid and pterin deaminases occur in extracellular and membrane-bound forms (10,11).…”