Differential effects of resact analogs on sperm respiration rates and cyclic nucleotide concentrations

Publisher Website
Google Scholar
Abstract
Analogues of resact (Cys-Val-Thr-Gly-Ala-Pro-Gly-Cys-Val-Gly-Gly-Gly-Arg-LeuNH2) were synthesized to determine whether or not a stimulation of sperm respiration could be obtained independent of elevations of cyclic nucleotide concentrations. Modification of the CO2-terminal leucine NH2 did not alter biological activity; however, substitution of the two cysteinyl residues by Ser or Tyr or methylation of the cysteinyl residues resulted in divergent relative potencies dependent on whether respiration rates or cyclic nucleotide concentrations were measured. [Ser1,Tyr8]resact, as an example, was approximately 40% as potent as resact at stimulating respiration rates but was 1% as effective as resact at causing cyclic GMP elevations. An NH2-terminal fragment (Cys-Val-Thr-Gly-Ala-Pro-Gly) neither stimulated respiration nor elevated cyclic nucleotide levels at concentrations up to 10 .mu.M whereas a CO2-terminal fragment (Cys-Val-Gly-Gly-Gly-Arg-LeuNH2) had approximately 20% of the respiration activity and 0.1% of the cyclic GMP elevating activity of resact. When the CO2- and NH2-terminal fragments were added simultaneously, however, cyclic nucleotide concentrations were elevated at the same relative concentrations as observed with resact. An analogue (125I-[Tyr1,Ser8]resact) was subsequently synthesized and used for receptor binding studies. Both the NH2-terminal and CO2-terminal fragments competed for binding, although they were 0.0004 and 0.025 times as effective as resact, respectively. However, in the presence of 1 .mu.M resact-(1-7), resact-(8-14) was almost as potent as resact in the competitive binding assay. The individual CO2- and NH2-terminal fragments appear to each bind to receptor, therefore, but the presence of one greatly potentiates the binding of the other peptide fragment. It also appears that cyclic nucleotide elevations can be separated from the stimulation of respiration, suggestive that increased concentrations of cyclic AMP or cyclic GMP do not mediate the effects of the peptides on respiration.