Phosphatidylethanolamine N-methyltransferase in human red blood cell membrane preparations. Kinetic mechanism.

The successive methylations of phosphatidylethanolamine to form phosphatidylcholine were measured using exogenously added intermediates and membrane preparations from human red blood cells. The addition of phosphatidylethanolamine resulted in no increase in methylation rate over that with endogenous substrate; however, the addition of monomethylphosphatidylethanolamine (PME) and dimethylphosphatidylethanolamine (PDE) markedly increased the reaction rate and allowed studies into the kinetic mechanism for the second and third methylation reactions. The data are consistent with catalysis of the last two methylations being by a single enzyme with a random Bi-Bi sequential mechanism. Analysis of PDE:phosphatidylcholine product ratios indicates that the enzyme can conduct multiple methylations of enzyme-bound phospholipid. The nature of the acyl chain (16:0 versus 18:1) of the phospholipid had only a small effect on the value of the kinetic constants. The maximal velocities obtained with the 18:1 substrate were less than 5% lower than those obtained with the 16:0 substrate. The Km values for the two phospholipids were 20-45 and 10-14 microM for the methylation of PME and PDE, respectively. The Km for S-adenosylmethionine (AdoMet) was 5-9 microM with PME and 4 microM with PDE as substrates. Depending on the acyl chain and the phospholipid, the Ki(AdoMet) varied from 8 to 19 microM, the Ki(PME) from 41 to 82 microM, and the Ki(PDE) from 35 to 61 microM. The Ki for S-adenosylhomocysteine (AdoHcy) was between 1.0 and 1.4 microM depending upon the variable substrate. The endogenous concentrations of PME and PDE in red blood cell membranes were estimated to be 0.49 and 0.24 mumol/liter packed cells, respectively. The product from the utilization of AdoMet, S-adenosylhomocysteine (AdoHcy), was shown to be a competitive inhibitor of its precursor, AdoMet, and a noncompetitive inhibitor of the two phospholipid substrates.

Aging and performance on the Luria-Nebraska neuropsychological battery.

The Luria-Nebraska Battery has been shown to be a valid measure of neuropsychological functioning in younger adults; however, little validity research with the elderly has been reported. Seventy-eight healthy elderly adults (mean age = 72.2 years) were tested with the Luria-Nebraska Battery and 92% of the profiles were found to be within normal limits. Significant age-related effects were also found on the Luria-Nebraska Battery. Similarly, the healthy elderly were divided into two age groups, young-old (60-74) and old-old (75+). Only the expressive speech and writing scales showed significant differences, and these were in the opposite direction one might expect, in that the old-old group performed significantly better. Additionally, the performance of 100 elderly brain damaged patients (mean age = 68 years) was examined and 86% of the profiles indicated the presence of brain damage. Overall, this study suggests that the Luria-Nebraska Battery may be a useful measure of neuropsychological functioning in elderly groups.

The bilateral isodense subdural hematoma on computerized tomographic scan.

Blood causes striking changes on computerized tomography. However, chronic subdural hematomas may become isodense with brain and therefore not visible directly. Midline and ventricular displacement, effacement of cortical sulci, narrowing of white matter on one side, and ventricular distortion should suggest a unilateral isodense process. Bilateral isodense subdural hematomas pose a major problem on computerized tomography since there are no indications of a mass lesion. A negative report could lull the clinician into a false sense of security. In these cases, general disappearance of sulci and considerable narrowing of ventricles are helpful findings. A particularly important and overlooked sign is an abnormally decreased bicaudate cerebroventricular index. Above all, a high degree of suspicion is vital.