The rate of exchange of actin‐bound nucleotide is decreased by a factor of about 20 when actin is complexed with DNAase I without affecting the binding constant of calcium for actin. Binding constants of DNAase I to monomeric and filamentous actin were determined to be 5 × 108 M−1 and 1.2 × 104 M−1 respectively. The depolymerisation of F‐actin by DNAase I appears to be due to a shift in the G‐Fequilibrium of actin by DNAase I. Inhibition of the DNA‐degrading activity of DNAase I by G‐actin is of the partially competitive type.

This is the largest cohort of prenatal del22q11.2 diagnoses. As in postnatally diagnosed cases, HDs were the most frequently observed abnormalities. However, thymus and kidney abnormalities and polyhydramnios should also be screened for in the prenatal diagnosis of del22q11.2. Only the time of diagnosis appeared to be strongly associated with the pregnancy outcome: the earlier the diagnosis, the higher the TOP rate.

No abstract

The kinetics of interaction of Ca2+ ions and nucleotides with G-actin have been investigated by making use of the enhancement of 1,N6-ethenoadenosine 5'-triphosphate (epsilon ATP) fluorescence on binding to actin, the enhancement of 2-[[2-[bis(carboxymethyl)amino]-5-methylphenoxy] methyl]-6-methoxy-8-[bis(carboxymethyl)amino]quinoline (Quin-2) fluorescence on binding to Ca2+, and the sensitivity of the fluorescence of an N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylenediamine (1,5-AEDANS) group on Cys-374 to metal ion binding. It is concluded that metal ion dissociation is the rate-limiting step in nucleotide dissociation (0.016 s-1 for Ca2+ at pH 7.2 and 21 degrees C) and that earlier conclusions that metal ion release is relatively fast and subsequent nucleotide release slow are incorrect. Results presented here and obtained by others on the metal ion concentration dependence of the effective rate of nucleotide exchange can be interpreted in the light of this conclusion in terms of a limiting rate which corresponds to that of metal ion release and an "apparent" dissociation constant for Ca2+ which is without direct physical significance. This apparent dissociation constant is more than 2 orders of magnitude greater than the real dissociation constant of Ca2+ from the Ca-actin-ATP complex, which was estimated to be 2 X 10(-9) M from a titration with Quin-2. Confirmation that the rate of Ca2+ release is rate limiting both in nucleotide dissociation reactions and in replacement of Ca2+ by Mg2+ was obtained with 1,5-AEDANS-actin, since both the replacement of Ca2+ by Mg2+ and the removal of Ca2+ to give the actin-ATP complex occurred at the same (slow) rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Preparations of chicken gizzard actin obtained from acetone-dried muscle powders prepared with various methods developed for skeletal muscle contain variable amounts of a P-actinin-like protein. This contamination is minimized if the procedure of muscle powder preparation includes washing with EDTA solution, and can be completely removed by gel filtration of G-actin on Sephadex G-100. The presence of 8-actinin activity manifests itself in an increased rate of actin polymerization, low filament lengths resulting in low reduced viscosity and enhanced ATP-splitting activity of actin polymer, and instability of the polymer in the absence of free ATP. Gizzard actin purified on a Sephadex G-100 column does not differ from rabbit skeletal muscle actin in its polymerization properties. The distinct property of gizzard actin is the instability of its G form in the absence of added Ca2+, indicating that the affinity of this cation for the single high-affinity site in gizzard actin is lower than in skeletal muscle actin.The procedures generally used to isolate muscle actin involve preparation of acetone-dried muscle powder from which monomeric actin is extracted and purified by reversible polymerization combined with ultracentrifugation. Earlier observations [l -31 indicated that polymerizable actin can be obtained from various vertebrate smooth muscles if the acetone-dried powder is prepared with the method of Carsten and Mommaerts [4]