en Biological Sciences Biological Sciences Original Research Original Research In this study, a commercially available kit in which propionylthiocholine is used as a substrate and a dibucaine solution to determine the variants of the enzyme were utilized. Plasma cholinesterase activity of 151 patients who underwent different operations was measured during a period of six months. The mean±SD of the enzyme activity in 150 patients was 4.39±1.17 kU/L .. Seventeen patients (11.2%) were found to have a serum cholinesterase activity of less th'an normal «3 Ku/L), but the period of apnea and the dibucaine number (DN) were found to be normal. Eight patients (5.3%) displayed an increased sensitivity to succinylcholine. Six of these patients (4%) were heterozygous for atypical enzyme with a mean period of apnea of 18.6 min (range 12-35 minutes) and had a dibucaine number of less than normal (‹80%). The DN of the other two patients (1.3%) who were homozygotes and had an exceedingly low atypical enzyme allele was 10% and 24%. The mean±SD DN value, in the 149 out of 151 patients was 84.41±2.91 %, and the DN ranged from 10-90% in these patients. The mean±SD apnea period in the 149 patients was 6.37±4.28 min. Plasma cholinesterase, Succinylcholine, Apnea, Dibucaine number (DN), Propionylthiocholine, Homozygote 105 109 http://mjiri.iums.ac.ir/browse.php?a_code=A-10-1-372&slc_lang=en&sid=1 MAHINDUKHT KEYHANI 20031947532846005141 20031947532846005141 Yes From the Department of Bio chemistry, Iran University of Medical Sciences, and the *Department of Anesthesiology, Tehran University of Medical Sciences, Tehran, Islamic Republic of Iran. ZAHID HUSSAIN KHAN 20031947532846005142 20031947532846005142 No