The chemoattractant folic acid binds reversibly to receptors at the surface of Dictyostelium cells. In undifferentiated cells (to.5) 6 x lo4 binding sites per cell with a Ko.S value of 1.5 x M were determined. The number of folic acid receptors per cell decreases slightly during cell development. In differentiated cells (tlo) 3.3 x lo4 binding sites per cell were estimated. The folic acid binding sites appear to be specific for folic acid and its derivatives. 2-Deamino-2-hydroxyfolic acid, 4-aminofolic acid (aminopterin), and 4-amino-10-methylfolic acid (aniethopterin) compete for the folic-acidbinding sites. Pterins, p-aniinobenzoic acid and glutamic acid show no significant competition for the folic-acid-binding sites. Nor do adenosine 3',5'-phosphate and guanosine 3',5'-phosphate affect binding of folic acid. The folic acid receptors appear to be distinct from the catalytic sites of the membrane-bound folic acid deaminase.Cells of Dictyostelium discoideum react chemotactically to cyclic AMP [l] as well as to folic acid and pterins [2,3] We investigated the binding of folic acid to cells of D. discoideum and provide evidence for the presence of folic acid receptors at the cell's surface. The number of folic acid receptors per cell decreases slightly during cell development. The receptors appear to be specific: folic acid derivatives compete for the folic-acid-binding sites, but pterins do not compete. Also cyclic nucleotides do not influence binding of folic acid. An account of this work was published in abstract form [ll].

Cells of Dictyostelium discoideum respond to extracellular cyclic AMP with marked changes in intracellular cyclic GMP levels and light scattering. In this work, defined temporal increases in cyclic AMP were produced by the continuous addition of cyclic AMP to agitated suspensions of cells; concomitant hydrolysis of cyclic AMP by the cells subsequently established a constant, steady state concentration. The cells responded to the initial increase in extracellular cyclic AMP with a rapid increase in the intracellular cyclic GMP concentration and a rapid decrease in light scattering. At cyclic AMP input rates of 0.5-5 nM x s -1, the fast reactions of cyclic GMP and light scattering had already relaxed while the cyclic AMP concentration in the cell suspension was still increasing. The cells responded to constant concentrations of cyclic AMP with constant elevated cyclic GMP concentrations and constant decreased levels of light scattering. Our results are consistent with the existence of two types of perception systems, one of which adapts to constant stimuli and one of which does not adapt.Cells of Dictyostelium discoideum respond chemotactically to cyclic AMP (1, 2) as well as to folio acid (3, 4) and pterin (4). During differentiation from the growth phase to the aggregation-competent state, the chemotactic sensitivity of the cells toward cyclic AMP increases (2) and that toward folic acid and pterins decreases (4). The presence of receptors for cyclic AMP (5-7) and for folio,acid (8, 9) at the cell surface has been demonstrated.

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.

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