A new
creatine analog,
1-carboxyethyl-2-iminoimidazolidine (
homocyclocreatine), has been synthesized and compared with other synthetic analogs of
creatine as a substrate for
creatine kinase under both in vitro and in vivo conditions. Reactivity with rabbit
muscle creatine kinase at 2 mM and pH 7.0 occurred in the order:
creatine greater than
cyclocreatine (1-carboxymethyl-2-iminoimidazolidine) greater than
N-ethylguanidinoacetate greater than N-propylguanidinoacetate greater than
guanidinoacetate greater than N-methyl-3-guanidinopropionate greater than 3-guanidinopropionate greater than
homocyclocreatine.
Homocyclocreatine was 10,000-fold less active than
creatine. In the reverse direction at 0.2 mM and pH 7.0:
creatine-P greater than
N-ethylguanidinoacetate-P greater than
cyclocreatine-P much greater than
homocyclocreatine-P.
Homocyclocreatine-P was 200,000-fold less active than
creatine-P. The phosphoryl group transfer potential of
homocyclocreatine-P was estimated to be 2 kcal/mol lower than that of
creatine-P. Chicks fed 5%
homocyclocreatine for 16 days synthesized and accumulated
homocyclocreatine-P in breast muscle (32 mumol/g wet wt), leg muscle (24 mumol/g), heart (7 mumol/g), intestine (8.5 mumol/g), and brain (2.4 mumol/g). During
ischemia homocyclocreatine-P was utilized by muscle much more slowly for the regeneration of
ATP than was
creatine-P or
cyclocreatine-P. Our results suggest that in tissues of
homocyclocreatine-fed animals subjected to a sudden large increase in work load or to
ischemia, the residual
creatine-P system would rapidly equilibrate with the adenylate system at the new lower cytosolic phosphorylation potential, whereas in the same cytosol the (
homocyclocreatine-P)/(
homocyclocreatine) ratio would exhibit a hysteresis or memory effect and reflect for a considerable period of time the earlier higher (
ATP)/(free
ADP) ratio rather than the actual lower (
ATP)/(free
ADP) ratio.