how to add primary key to existing table with multiple primary keys

Hi there,

I am trying to add another primary key to an existing table with two other primary keys.  I got the following error when I tried this command:

alter table extprobe2tissue ADD primary key (expid);

ERROR:  multiple primary keys for table "extprobe2tissue" are not allowed

The details of the extprobe2tissue table is:

\d extprobe2tissue;

          Table "public.extprobe2tissue"

   Column    |          Type          | Modifiers

-------------+------------------------+-----------

 probeid     | integer                | not null

 tissueid    | integer                | not null

 value       | double precision       | not null

 entrydate   | date                   | not null

 description | character varying(200) |

 qval        | double precision       |

 expid       | integer                | not null

Indexes:

    "extprobe2tissue_pkey" PRIMARY KEY, btree (probeid, tissueid)

    "extprobe2tissue_probeid" btree (probeid)

    "extprobe2tissue_tissueid" btree (tissueid)

Foreign-key constraints:

    "extprobe2tissue_tissueid_fkey" FOREIGN KEY (tissueid) REFERENCES tissue(tissueid)

    "extprobe2tissue_probeid_fkey" FOREIGN KEY (probeid) REFERENCES extprobe(probeid)

Any suggestion would be much appreciated.

Thanks!