Our investigations of metal metabolism in mussels involved 
studies of both laboratory and field populations. Standard 
biochemical methods were used to separate metal-binding 
proteins. The mussel tissue that was used was the digestive 
gland, which is known to concentrate metals and is homologous to 
the liver of mammals. Digestive glands from 25 mussels were 
pooled, homogenized, centrifuged at 100,000 x g, and then an 
aliquot of the supernatant fluid was applied to a gel permeation 
chromatography column. The column effluent was monitored for 
absorbance in the UV region and collected in a fraction 
collector. Sample fractions were analyzed for each metal. Two 
metal peaks are generally found. The first peak represents 
metals associated with high-molecular-weight (HMW) proteins and 
the second peak with low-molecular-weight (LMW) proteins. The 
HMW proteins include metalloenzymes that are necessary for 
normal metabolic activities and are considered to be the sites 
of toxic action of metals; the LMW proteins include metallothio- 
neins that are considered to be the sites of detoxification. 
The changes in the amounts of copper associated with these 
two sizes of proteins are shown for digestive glands of mussels 
that had been exposed for three weeks to 25, 50, and 75 pg Cu/L 
(Figure 2). The quantities of copper associated with both the 
LMW and HMW proteins were greater in those exposed to copper. 
However, whereas the amount in the HMW proteins increased with 
exposure concentration, that associated with the LMW proteins 
was highest in those that had been exposed to 50 ug Cu/L. These 
results indicate that exposure to 75 pg Cu/L for three weeks was 
not well tolerated. This was indicated also from the mortality 
data that showed a large percentage of mortality in the group 
exposed to 75 pg Cu/L. Although the mortality was correlated 
with the amount of Cu associated with the HMW peak (Figure 3), 
it does not establish a cause-effect relationship. 
A second experiment in which mussels were exposed to 25 ug 
Cu/L for 12 weeks was performed. The quantities of metals as¬ 
sociated with the HMW and LMW proteins was quantified; the a- 
mount associated with the HMW proteins continued to increase 
with time whereas that associated with the LMW proteins MT ap¬ 
peared to plateau (Figure 4). The presence of a plateau indi¬ 
cates that the quantities of MT that are produced are limited, 
which, in turn, implies that the detoxification provided by this 
process is also limited. 
It has been established that pre-exposure to low concen¬ 
trations of metals may result in the induction in the synthesis 
of MT; this phenomenon may account for the large increase be¬ 
tween 3- and 6-week samples in the amount of copper associated 
with LMW proteins. It has also been established that increased 
concentrations of MT results in increased tolerance to exposure 
to additional metals. In our experiments, the possibility of 
increased tolerance to copper was not examined. 
Ill 
