The elasticity of spectrin-actin gels at high protein concentration

Human erythrocyte spectrin of high purity was studied alone and mixed with rabbit skeletal actin by dynamic rheometry as a function of protein concentration at pH 7.4 and 24 degrees C. Pure spectrin had a very low storage modulus, G', increasing slightly with increase in protein concentration (approximately 3 dynes/cm at 25 mg/ml). In contrast, unpurified cytoskeletal extracts containing spectrin, actin, and band 4.1 showed a marked concentration dependence for G', increasing to 150 dynes/cm at 20 mg/ml. Mixtures of purified spectrin and skeletal actin at a weight ratio of 4:1 also showed G' markedly dependent on concentration (approximately 150-200 dynes/cm at 20 mg/ml). Maximum elasticity of spectrin-actin gels occurred at a molar ratio of actin monomers to spectrin tetramers of 14:1. We conclude that the reconstituted in vitro spectrin-actin network consists of actin fibers cross-linked by spectrin tetramers at regular intervals. The gel is rapidly reformed after mechanical disruption or thermal collapse, indicating that the polymer fibers are in equilibrium with the constituent monomers.

Effect of recent food on estimation of high-density lipoprotein and total cholesterol in normal subjects

Magnesium/phosphotungstic acid was confirmed as specific for precipitation of chylomicrons, low-density and very low density lipoprotein particles in serum from healthy volunteers who were either fasting or who had eaten egg yolks or a salmon sandwich containing Flora margarine 2 hours previously. In eight subjects who ate salmon sandwiches that contained butter or Flora margarine, neither the total cholesterol nor the high-density lipoprotein (HDL) cholesterol differed significantly from the fasting level over 7 hours. In eight subjects, egg yolks also did not alter significantly either the total serum cholesterol or the HDL cholesterol at 1/2, 1, 1 1/2, 2, 5, and 7 hours after eating. Patients may be screened for HDL cholesterol and total cholesterol estimation when not fasting. However, individual subjects showed variation in their total cholesterol and, particularly, HDL cholesterol over a three-month period. Replicate analysis of HDL cholesterol may be required for its complete assessment.