1983•Unpublished venueRequires access
Mechanisms ofALA-Dinhibition byleadandofits restoration byzincanddithiothreitol
Tadashi Sakai, Yukiko Kunugi, Koichi USHIO
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Abstract
acid dehydratase) activity byleadinvitro, itisnecessarytopreincubate theenzyme fraction withleadionsandtheHb fraction (factors) together. Thecombination oftwoofthethree (ALA-Dfraction, leadacetate, andHbfraction) inthepreincubation hasonly asmall effect on theactivity. Leadpreincubated withALA-DandHb fractions doesnotalter theaffinity oftheenzyme forthesubstrate, suggesting that thesubstrate canbind totheenzyme molecules (non-competitive inhibition). The restoration ofactivity byzincwithdithiothreitol isassociated withtheremoval ofleadfrom ALA-Dfraction proteins towhich ithasboundinvivoandinvitro. Themodeoftheinhibitory action oftin on ALA-Dissimilar tothatofleadbecause theinhibition isintensified bythe addition ofHbfraction andisrestored byheating. Theinhibition concentration is,however, higher thanthat oflead. Ofthethree methods recovering decreased activity, heating isthemost specific todetect inhibition bylead. Erythrocyte ALA-D(8-aminolaevulinic acid dehydratase) activity iswidely usedfortheevaluation of lead exposure orfortheroughestimation ofleadin blood that iscurrently regarded asthemostsensitive index ofleadexposure. Sakai etal'haveshownthat thedecrease inALA-Dactivity isanindicator ofthe extent ofleadsaturation ofALA-Dfraction proteins which havethehighest affinity forleadamong erythrocyte constituents. Nevertheless, themechanismsofALA-Dinhibition byleadinthefraction havenotbeenelucidated, although someerythrocytefactors areknowntobeinvolved intheinhibition.2 Depressed ALA-Dactivity inlead-exposed subjects isrestored byheating theenzymesolution3 4or byadding SH-compounds56 orzincions'8 tothe reaction mixture. Manyworkers5 9-16havetried to evaluate lead exposure, using these agents orprocedures recovering theactivity. Theextent ofrestoration inactivity (activity ratio orVHE)iswellcorrelated withblood leadconcentrations, andthecorrelation isbetter thanthatbetween ALA-Dactivity itself andbloodleadconcentrations. Thisisprobablybecause thenormalrangeofactivity ratio is muchnarrower thanthat ofALA-Dactivity itself,