TI - Cyclocreatine Inhibits Phagocytosis of Zymosan and COZ . AB - On the basis of this premise , we wondered whether the CK-driven ATP-PCr exchange reaction could be directly or indirectly coupled to the process of particle ingestion . Initially , we chose a pharmacological approach to modulate activity of the entire cellular pool of CK , applying Cr as a stimulating SUBstrate or cyclocreatine ( cCr ) as a reversible inhibitor of the CK reaction . RAW 264.7 cells were preincubated with 5 mM cCr or Cr prior to the phagocytosis assay and cells were then challenged with differentially opsonized and fluorescently labeled phagocytic targets . After 30 min , phagocytic activity was quantified by determining the mean fluorescence intensity of ingested particles in the different cells by fluorescence-activated cell sorter ( FACS ) analysis . In Figure 5 , data are shown that are normalized to values for nontreated cells . Phagocytosis of nonopsonized zymosan was slightly affected by cCr treatment , yielding an efficiency value of 79% + - 9% ( p lt 005 ) , whereas Cr supply had no significant effect ( 102% + - 9% ) ( Figure 5A ) . Interestingly , cCr inhibition decreased phagocytosis of COZ to a much lower level , 37% + - 6% ( p lt 0005 ) of that of nontreated cells , whereas Cr addition had no significant effect ( value 84% + - 8% ) ( Figure 5B ) . In contrast , Cr and cCr addition had no significant effect on phagocytosis of IgG-opsonized zymosan , with 97% + - 5% and 88% + - 9% for Cr - and cCr-treated cells , respectively ( Figure 5C ) . In order to verify that this difference was indeed due to differential effects on CR3 and Fc-gammaR receptor -mediated activities and cannot be attributed to interference with other pathways , we performed receptor -blocking experiments . Capture uptake of the two different types of opsonized zymosan appeared indeed specific for the anticipated receptors (Figure S1) . To study this point further , we also tested phagocytic activity on complement - and IgG-opsonized polystyrene beads , which lack obvious surface ligands such as mannose or beta-glucan groups , and therefore form "clean" targets . Interestingly , uptake of complement-opsonized beads was again inhibited by cCr ( 56 + - 4% of control ) , whereas IgG -mediated phagocytosis remained unaffected ( 95 + - 2% of control ; Figure S2 ) . Thus , although CK mobilization is seemingly a default event in all types of phagocytosis ( Figure 4 ) , it may only selectively contribute to the efficiency of phagocytic ingestion of nonopsonized or complement-opsonized particles ( Figures 5 and S2 ) . A similar situation was recently reported for the cytoskeletal actin-binding protein talin , whose functional role in phagocytic uptake appeared selectively coupled to CR3 , but which accumulates in phagosomes formed around IgG - and C3-opsonized particles [38] .