US EPA

193000 

Journal Article 

Acetominophen and phenol: Substrates for both a thermostable and thermolabile form of human platelet phenol sulfotransferase 

Reiter, C; Weinshilboum, RM 

1982 

Yes 

Journal of Pharmacology and Experimental Therapeutics
ISSN: 0022-3565
EISSN: 1521-0103 

221 

1 

43-51 

English 

6950087 

http://jpet.aspetjournals.org/content/221/1/43.full.pdf
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Human platelet homogenates were used to test the hypotheses
that multiple forms of platelet phenol sulfotransferase (PST)
with differing physical properties and substrate specificities
might exist and that some compounds might serve as substrates
for more than one form of platelet PST activity. PST
thermal stability was determined in platelet homogenates preheated
for 1 5 mm at temperatures varying from 37-49#{176}C.
When dopamine was used as a substrate, 50% of the enzyme
activity was inactivated at approximately 39.5#{176}C. However,
PST activity measured with 1 OOjıM phenol as substrate was
50% inactivated at approximately 44#{176}C.Acetaminophen (16
mM) behaved as if it might serve as a substrate for both the
thermolabile and the thermostable PST activities. Kinetic experiments
were performed with platelet homogenates and
phenol showed a biphasic double inverse plot with two apparent
Km values of 1 6 and 638 ıtM. Thermal treatment of platelet
homogenates that caused inactivation of the thermolabile PST
activity resulted in loss of the portion of the double inverse plot
that gave the higher apparent Km value. These results suggested
that, depending on the concentration tested, phenol
might serve predominantly as a substrate for either the thermolabile
or the thermostable form of human platelet PST. When
the hypothesis that phenol might serve as a substrate for either
the thermolabile or thermostable form of human platelet PST
was tested further, it was found that the enzyme activity measured
with 62.5 ıtM phenol was thermostable whereas that
measured with 4 mM phenol was thermolabile. PST activity
measured with 500 .tM phenol was intermediate in thermal
stability. Thermal inactivation studies performed by heating
platelet homogenates at 43#{176}Cfor variable time periods mdicated
that acetaminophen was also able to serve as a substrate
for both the thermolabile and thermostable forms of human
platelet PST activity. The results of experiments in which PST
activity was measured in platelet homogenates from 23 individual
subjects were also compatible with the conclusion that
phenol and acetaminophen were able to serve as substrates
for both the thermostable and thermolabile forms of human
platelet PST activity and that the activities of these two forms
of the enzyme were regulated independently.