Try using the key keyword with sorted().
sorted([('abc', 121),('abc', 231),('abc', 148), ('abc',221)], key=lambda x: x[1])
key should be a function that identifies how to retrieve the comparable element from your data structure. In your case, it is the second element of the tuple, so we access [1].
For optimization, see jamylak's response using itemgetter(1), which is essentially a faster version of lambda x: x[1].