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The superficial, rather unreasoned answer is "Yes." There is a REST API endpoint to lookup lead(s) by email.
The realistic answer is that you'll be introducing a massive Denial of Service vulnerability if you expose individual API calls in this way. The only way to roll out something like this professionally is to enforce rate limits using a robust algorithm, plus daily caps to leave space for other integrations -- but even with those controls you may not have enough to service legit end users.
A more appropriate way to reapproach this goal is mirroring your Mkto database, via periodic batches, to your own database. Then offer that second db for real-time lookups. The Marketo API is good for batch/bulk operations, not one-at-a-time use.